Zinc oxide varistor and process for the production thereof

ABSTRACT

A product and process of making such product in which a varistor is formed by diffusing lead borosilicate-type glass, into a surface of a fired or sintered zinc oxide substrate, i.e., &#34;varistor element,&#34; during formation of an electrode on the surface of the substrate. Typically, an electrode paste or material, comprising a mixture or lead borosilicate-type glass frit and Ag powder, is applied to the substrate and provides the lead borosilicate-type glass for diffusing into the substrate. The improvement is that the lead borosilicate-type glass frit for the electrode paste or material comprises a mixture of PbO, B 2  O 3 , SiO 2  and at least one metal oxide selected from the group consisting of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetium oxide.

TECHNICAL FIELD

The present invention relates to a zinc oxide varistor used forprotecting various kinds of electronic instruments from unusually highvoltages, and a process for producing the same.

BACKGROUND TECHNIQUES

Recently, there has been rapidly developed a high level integration ofcontrol circuits in instruments for general use and industry.

When an extraordinarily high voltage (surge) is applied to electronicparts of semiconductors used in such control circuits, such parts may bedestroyed. Accordingly, it becomes indispensable to take acountermeasure to meet the situation. As such a counterplan, varistorsare generally employed. Among the rest, the zinc oxide varistor iswidely available for the protection of various kinds of electronicinstruments from unusually high voltages because the zinc oxide varistorhas an excellent voltage nonlinearity and surge absorbing ability.

Hithertofore, there has been widely known a zinc oxide varistor providedwith at least two electrodes on the surface of varistor element havingzinc oxide as its main component. Further, materials for saidelectrodes, are disclosed in, for example, Patent Application Kokai SHO62-290104 Official Gazette, etc., whose content is as follows:

Electrode material for a zinc oxide varistor was produced by the processwherein 5.0% by weight of a lead borosilicate glass powder composed of50.0-85.0% by weight of PbO, 10.0-30.0% by weight of B₂ O₃ and 5.0-25.0%by weight of SiO₂ was weighed out and then said powder together with Agpowder (65.0% by weight) were milled in a vehicle (30.0% by weight), inwhich ethyl cellulose was dissolved in butyl carbitol, to obtain asilver paste which is the electrode material.

And then said electrode material was applied onto a surface of a firedvaristor element and heated to form an electrode.

Although the above zinc oxide varistor is excellent in voltagenonlinearity as mentioned above, further improvement in the voltagenonlinearity has been sought due to the desire of energy-saving andefficiency increase in the zinc oxide varistor.

Thus, responding to the above requirements, the present invention aimsto provide a zinc oxide varistor further improved in voltagenonlinearity.

DISCLOSURE OF THE INVENTION

In order to accomplish such an objective, according to the presentinvention, the following lead borosilicate-type glass was diffused intoa fired varistor element from its surface, said lead borosilicate-typeglass containing at least one metal oxide selected from cobalt oxide,magnesium oxide, yttrium oxide, antimony oxide, manganese oxide,tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide,neodymium oxide, samarium oxide, europium oxide, gadolinium oxide,terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thuliumoxide, ytterbium oxide and lutetium oxide.

When the above constitution is adopted, it follows that there isinterposed at particle boundaries between zinc oxide particles composinga varistor element, the chemical elements composing a leadborosilicate-type glass containing at least one metal oxide selectedfrom cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodymium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide.

As a result, resistance values of the particle boundaries between zincoxide particles will become higher, and a leakage current runningbetween electrodes until reaching a varistor voltage becomes much lower.In conclusion, zinc oxide varistor improved in voltage nonlinearity canbe obtained.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a front view showing one of the working examples of the zincoxide varistor of the present invention. FIG. 2 is a sectional view ofFIG. 1, and FIG. 3 is a front view showing varistor element of the zincoxide varistor shown in FIG. 1.

BEST MODES FOR CARRYING OUT THE INVENTION

One of the working examples of the present invention is explained withreference to the drawings as follows:

FIG. 1 and FIG. 2 show one of the working examples of the presentinvention. In the drawings, 1 is a disk-shape varistor element which is13 mm in diameter and 1.5 mm in thickness.

On both surfaces of this varistor element 1, electrodes 2 are bakedthereto as shown in FIG. 3.

The electrodes 2 are also disk-shape of 10 mm in diameter, and anoutside periphery part of varistor 1 projects out and around the wholecircumference of the electrodes.

In addition, upper end of lead wire 3 is fixed onto each electrode 2 bysoldering.

Under said state, the outside periphery of varistor element 1 is coatedwith an epoxy-type insulative resin 4. As shown in FIG. 1, only thelower end of the lead wire is drawn out to the outside of the insulativeresin 4.

It should be noted that the present working example is characterized bythe material of electrode 2. That is, the present working example usedthe material formulated by milling a lead borosilicate-type glass fritinto a Ag paste. This will be explained in detail hereinunder.

(Working Example 1)

At first, preparation of the glass frit will be mentioned. According tothe composition table of the following Table 1, PbO, B₂ O₃, SiO₂ and Co₃O₄ were weighed each in a given amount, and then they weresimultaneously mixed and ground in a ball-mill. Thereafter, saidadmixture was fused in a platinum crucible at a temperature condition of1000° C.-1500° C., and then quenched to be glassified. The obtainedglass was roughly ground, which was followed by fine milling in aball-mill to obtain a lead borosilicate-type glass frit. On the otherhand, as a lead borosilicate glass frit of conventional example, a glassfrit composed of 70.0% by weight of PbO, 15.0% by weight of B₂ O₃, and15.0% by weight of SiO₂ was formulated in a similar manner. The glasstransition point (Tg) of each glass prepared as above was as shown inthe following Table 1. Hereupon, the glass transition point (Tg) wasdetermined by using a thermal analysis apparatus.

                  TABLE 1                                                         ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Co.sub.3 O.sub.4                                                                    (°C.)                           ______________________________________                                         A*       70       15     15     0     405                                    B         69.9     15     15     0.1   405                                    C         60       15     15     10    420                                    D         45       15     15     25    465                                    E         40       15     15     30    475                                     F*       35       15     15     35    490                                     G*       30       34.9   35     0.1   545                                    H         40       29.9   30     0.1   520                                    I         89.9     5       5     0.1   315                                     J*       60       0      15     25    445                                    K         55       5      15     25    450                                    L         50       30     15     5     480                                     M*       40       40     15     5     500                                     N*       60       15      0     25    440                                    O         55       15      5     25    445                                    P         50       15     30     5     495                                     Q*       40       15     40     5     515                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

Then, 5.0% by weight of the lead borosilicate-type glass frit wasweighed which was followed by milling in the above-mentioned Ag paste(65% by weight of Ag powder was dissolved into 30% by weight of avehicle in which ethyl cellulose is dissolved into butyl carbitol) toproduce electrode material for a zinc oxide varistor.

In order to evaluate the electrode material for zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1 in FIG. 3) (a disk-shape of 13 mm in diameter and1.5 mm in thickness) was provided, said sintered-body consisting ofbismuth oxide (Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO₂),nickel oxide (NiO) and titanium oxide (TiO₂) respectively in 0.5 mole %,and antimony oxide (Sb₂ O₃), and chromium oxide (Cr₂ O₃) respectively in0.1 mole %, and 0.005 mole % of Al₂ O₃, the rest being zinc oxide (ZnO).On both surfaces of said sintered-body, an electrode material for zincoxide varistor was screen-printed to be 10 mm in diameter, and thenbaked at 800° C. for 10min. to form electrodes 2 as shown in FIG. 3.After lead wires 3 indicated in FIG. 2 were soldered thereon, the outerperiphery was coated with insulating resin 4 to obtain a sample. It isnoted that when the above electrode material is applied onto a surfaceof the sintered-body (varistor element 1) and then heated, a leadborosilicate-type glass in the electrode material, which contains cobaltoxide will penetrate into the varistor element 1, thereby exerting itseffect as under-mentioned.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀ μArepresenting voltage nonlinearity), surge current resistancecharacteristic and high temperature load life performance are shown inthe following Table 2. The above voltage ratio (voltage nonlinearity)was obtained through determination using a direct current constantcurrent electric source. Further, surge current resistancecharacteristic was obtained by determining a variation ratio of varistorvoltage (V₁ mA) occurring when an impact current of 8/20 μS standardwaveform and 2500 A crest value was applied two times in the samedirection. It is preferred that such a value is less than that inconventional example A. Further, high temperature load life performancewas obtained by determining a variation ratio of varistor voltage (V₁mA) after 1000 hrs. when direct current voltage corresponding to 90% ofsample varistor voltage was applied between lead terminals 3 at anenvironment temperature of 125° C. Such a value is preferably lower thanthat in conventional example A. The number of samples was 10 per lot.

Further, the above voltage ratio (V₁ mA /V₁₀ μA) indicates voltagenonlinearity. When the voltage ratio is less than that in conventionalexample A, a leakage current up to reaching a varistor voltage willbecome lower than conventional one. That is, V₁ mA represents a voltage(varistor voltage) when 1 mA current runs between electrodes 2.Likewise, V₁₀ μA represents a voltage when 10 μA current runs betweenelectrodes 2. A small value of V₁₀ μA is not preferable because a highleakage current runs from a low voltage.

                                      TABLE 2                                     __________________________________________________________________________                     Surge current resistance                                                                         High temperature load life                                 characteristic ΔV.sub.1 mA (%)                                                             performance ΔV.sub.1 mA (%)         Sample                                                                            Designation  Direction same as                                                                      Direction reverse to                                                                    Direction same as                                                                      Direction reverse to             No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           that of current                                                                        that of current                                                                         that of current                                                                        that of current                  __________________________________________________________________________    1    A*   1.83   -22.3    -28.9     -3.9     -10.8                            2   B     1.52   -10.9    -18.0     +1.5     -2.9                             3   C     1.36   -9.7     -14.5     +1.4     +0.9                             4   D     1.28   -5.9     -8.3      +2.0     +1.1                             5   E     1.32   -8.8     -11.9     +2.1     +1.1                             6    F*   1.71   -16.7    -21.7     +1.2     -1.7                             7    G*   1.51   -16.2    -23.5     +1.3     -2.4                             8   H     1.46   -12.8    -17.3     +2.2     +0.3                             9    I*   1.38   -25.5    -36.9     -10.5    -20.8                            10   J*   1.30   -20.4    -26.0     +0.8     -2.8                             11  K     1.32   -10.2    -16.4     +1.7     +0.1                             12  L     1.39   -11.5    -19.1     +1.8     +0.2                             13   M*   1.36   -18.4    -26.3     +1.9     -0.2                             14   N*   1.32   -21.0    -27.8     +1.1     -3.7                             15  O     1.34   -11.3    -17.2     +1.8     +0.4                             16  P     1.36   -10.1    -18.2     +1.0     +0.2                             17   Q*   1.45   -20.5    -28.4     +0.9     +0.1                             __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 1 and 2 the influence onvoltage ratio (voltage nonlinearity), surge current resistancecharacteristic and high temperature load life performance by Co₃ O₄content contained in a lead borosilicate-type glass frit in an electrodematerial for a zinc oxide varistor. As compared with the leadborosilicate glass of the conventional example containing no Co₃ O₄(Designation of glass: A in Table 1), the composition systems having Co₃O₄ content of 0.1% by weight or more are improved in voltage ratio(voltage nonlinearity) but those having Co₃ O₄ content of more than30.0% by weight or more will deteriorate voltage nonlinearity and surgecurrent resistance characteristic. Accordingly, it is a necessarycondition that lead borosilicate glass in an electrode material for zincoxide varistor is a composition system containing at least 0.1-30.0% byweight of Co₃ O₄.

On the other hand, since surge current resistance characteristic andhigh temperature load life performance are affected by contents of PbO,B₂ O₃ and SiO₂ in addition to Co₃ O₄ content, these compositions arerequired to be considered. Therefore, influence on surge currentresistance characteristic and high temperature load life performance byconstitution components of lead borosilicate-type glass contained in anelectrode material for a zinc oxide varistor will be considered on thebasis of Tables 1 and 2. Glass of a composition system having PbOcontent less than 40.0% by weight has a higher glass transition point(Tg in Table 1) and too small a fluidity of the glass, which results ina deteriotated solder-wetness of the glass. Contrarily, glass of acomposition system having PbO content of more than 80.0% by weight has alower glass transition point and too high a fluidity of the glass, whichresults in a lower adhesion strength of electrode 2 onto varistorelement 1, this fact leads to a lack of reliability. In a compositionsystem having B₂ O₃ content of less than 5.0% by weight, surge currentresistance characteristic becomes inferior. On the other hand, in acomposition system having B₂ O₃ content of more than 30.0% by weight,surge current resistance characteristic is also deteriorated. In acomposition system having SiO₂ content of less than 5.0% by weight,surge current resistance characteristic is also lowered. In acomposition system having SiO₂ content of more than 30.0% by weight,surge current resistance characteristic will also become lowered.

From the above results, it is understandable that a composition of glasscomponents of an electrode material for a zinc oxide varistor is optimumin a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂ O₃,5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of Co₃ O₄.

Although lead oxide, boron oxide, silicon oxide and cobalt oxide wereused, as material of lead borosilicate-type glass, in the forms of PbO,B₂ O.sub. 3, SiO₂ and Co₃ O₄, respectively in the present workingexample, it was confirmed that similar characteristics could also havebeen obtained by using the other oxide forms. Further, the presentworking example referred only to the case in which leadborosilicate-type glass content in electrode material for a zinc oxidevaristor was 5.0% by weight. However, so far as said content is within1.0-30.0% by weight, no change is seen in the effect of the presentinvention. Furthermore, the zinc oxide varistor of system consisting ofZnO, Bi₂ O₃, Co₃ O₄, MnO₂, NiO, TiO₂, Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ was usedas a sintered varistor element 1 for evaluation. However, even when theelectrode material for a zinc oxide varistor according to the presentinvention is applied to a zinc oxide varistor containing Pr₆ O₁₁, CaO,BaO, MgO, K₂ O, SiO₂, etc., no change is seen in effect.

(Working Example 2)

Hereinunder, detailed explanation is made for the second working exampleof the present invention.

At first, the description refers to formulation of glass frit to beincorporated to electrode material for zinc oxide varistor. According tothe composition list of the following Table 3, PbO, B₂ O₃, SiO₂ and MgOweighed each in a given amount were mixed and simultaneously ground in aball mill, and then fused under a temperature condition of 1000°C.-1500° C. in a Pt-crucible, which was followed by quenched to beglassified. The thus-obtained glass was roughly crushed and then finelymilled in a ball mill to obtain lead borosilicate-type glass frit. Also,glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B₂O₃ and 15.0% by weight of SiO₂ was prepared by a similar procedure, as aconventional example of lead borosilicate glass. The glass transitionpoint (Tg) of the thus-obtained glass is shown in the following Table 3.Herein, the glass transition point (Tg) was determined using a thermalanalysis apparatus.

                  TABLE 3                                                         ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            MgO   (°C.)                           ______________________________________                                         A*       70       15     15     0     405                                    B         69.9     15     15     0.1   405                                    C         60       15     15     10    420                                    D         50       15     15     20    410                                    E         40       15     15     30    420                                     F*       40       10     10     40    410                                     G*       30       34.9   35     0.1   545                                    H         40       29.9   30     0.1   520                                     I*       89.9     5       5     0.1   315                                     J*       65       0      15     20    390                                    K         60       5      15     20    395                                    L         50       30     15     5     470                                     M*       40       40     15     5     490                                     N*       65       15      0     20    410                                    O         60       15      5     20    415                                    P         50       15     30     5     490                                     Q*       40       15     40     5     510                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

Then, the lead borosilicate-type glass frit was weighed by 5.0% byweight, which was followed by milling in the above-mentioned Ag paste(65% by weight of Ag powder was dissolved into 30% by weight of avehicle, in which ethyl cellulose is dissolved into butyl carbitol) toproduce electrode material for a zinc oxide varistor.

In order to evaluate the electrode material for a zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1) (a disk-shape of 13 mm in diameter and 1.5 mm inthickness) was provided, said sintered-body consisting of bismuth oxide(Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO.sub. 2), nickeloxide (NiO) and titanium oxide (TiO₂) respectively in 0.5 mole %, andantimony oxide (Sb₂ O₃) and chromium oxide (Cr₂ O₃) respectively in 0.1mole %, and 0.005 mole % of Al₂ O₃, the rest being zinc oxide (ZnO). Onboth surfaces of said sintered-body, an electrode material for zincoxide varistor was screen-printed to be 10 mm in diameter, and thenbaked at 800° C. for 10 min. to form electrodes 2 and then lead wires 3were soldered thereon, and thereafter the outer periphery was moldedwith insulative resin 4 to obtain a sample.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀ μA)and limit voltage ratio and surge current resistance characteristic areshown in the following Table 4. Herein, the voltage ratio and limitvoltage ratio were obtained through determination using a direct currentconstant current electric source. Further, the surge current resistancecharacteristic was obtained by determining a variation ratio of varistorvoltage (V₁ mA) occurring when an impact current of 8/20 μS standardwaveform and 2500 A crest value applied two times in the same direction.The number of samples was 10 per lot.

                                      TABLE 4                                     __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.5 A /V.sub.1 mA                                                                   that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.83   1.93      -22.3    -28.9                                     2   B     1.50   1.77      -11.2    -18.3                                     3   C     1.32   1.66      -9.6     -15.4                                     4   D     1.24   1.51      -5.3     -7.8                                      5   E     1.35   1.71      -7.4     -11.7                                     6    F*   1.56   1.85      -16.6    -21.8                                     7    G*   1.51   1.76      -17.8    -24.1                                     8   H     1.45   1.74      -11.4    -18.4                                     9   I     1.39   1.88      -26.4    -33.8                                     10   J*   1.31   1.59      -20.7    -25.1                                     11  K     1.30   1.56      -10.3    -15.8                                     12  L     1.37   1.66      -11.4    -18.7                                     13   M*   1.39   1.68      -19.6    -26.8                                     14   N*   1.28   1.59      -17.1    -25.8                                     15  O     1.31   1.58      -11.0    -16.4                                     16  P     1.38   1.65      -10.8    -17.9                                     17   Q*   1.43   1.66      -21.4    -29.7                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 3 and 4, the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by MgO content contained ina lead borosilicate-type glass frit in an electrode material for a zincoxide varistor. As compared with the lead borosilicate glass of theconventional example containing no MgO, the composition systems havingMgO content of 0.1% by weight or more are improved in voltage ratio(voltage nonlinearity) but those having MgO content of more than 30.0%by weight will deteriorate in limit voltage characteristic and surgecurrent resistance characteristic. Accordingly, it is a necessarycondition that a lead borosilicate-type glass in an electrode materialfor a zinc oxide varistor is a composition system containing at least0.1-30.0% by weight of MgO.

On the other hand, since the limit voltage ratio characteristic (V₅ A/V₁ mA) and surge current resistance characteristic are affected bycontents of PbO, B₂ O₃ and SiO₂ in addition to MgO content, thesecompositions are required to be considered. Therefore, influence onlimit voltage ratio characteristic and surge current resistancecharacteristic by constitution components of lead borosilicate glasscontained in an electrode material for zinc oxide varistor will beconsidered on the basis of Tables 3 and 4. Glass of a composition systemhaving PbO content of less than 40.0% by weight has a higher glasstransition point and too little a fluidity of glass, which result in alower solder-wetness of glass. Contrarily, glass of a composition systemhaving PbO content of more than 80.0% by weight has a lower glasstransition point and too great a fluidity of glass, which results in alower adhesion strength of an electrode. Therefore, this fact leads tolack of reliability. In a composition system having B₂ O₃ content ofless than 5.0% by weight, surge current resistance characteristicbecomes inferior. On the other hand, in a composition system having B₂O₃ content of more than 30.0% by weight, surge current resistancecharacteristic is also deteriorated. In a composition system having SiO₂content of less than 5.0% by weight, surge current resistancecharacteristic is also deteriorated. In a composition system having SiO₂content of more than 30.0% by weight, surge current resistancecharacteristic will also become deteriorated.

From the above results, it is understandable that composition of glasscomponents of electrode material for zinc oxide varistor is optimum tobe in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of MgO.

Although lead oxide, boron oxide, silicon oxide and magnesium oxide wereused, as materials of lead borosilicate-type glass, in the forms of PbO,B₂ O₃, SiO₂ and MgO, respectively in the present working example, it wasconfirmed that the similar characteristics could have also been obtainedby using the other oxide forms. Further, the present working examplereferred only to the case in which the lead borosilicate-type glasscontent in electrode material for zinc oxide varistor was 5.0% byweight. However, so far as said content is within 1.0-30.0% by weight,no change is seen in the effect of the present invention. Furthermore,the zinc oxide varistor of a system consisting of ZnO, Bi₂ O₃, Co₃ O₄,MnO₂, NiO, TiO₂, Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ was used as a sintered-bodyfor evaluation. However, even when the electrode material for the zincoxide varistor according to the present invention is applied to a zincoxide varistor containing Pr₆ O₁₁, CaO, BaO, MgO, K₂ O, SiO₂, etc., nochange is seen in effect.

(Working Example 3)

Hereinunder, detailed explanation is made for the third working exampleof the present invention.

At first, the description refers to formulation of glass frit to beincorporated to electrode material for zinc oxide varistor. According tothe composition list of the following Table 5, PbO, B₂ O₃, SiO₂ and MnO₂each weighed in a given amount were mixed and simultaneously ground in aball mill, and then fused under a temperature condition of 1000°C.-1500° C. in a Pt-crucible, which was followed by quenching to beglassified. The thus-obtained glass was roughly crushed and then finelymilled in a ball mill to obtain lead borosilicate-type glass frit. Also,glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B₂O₃ and 15.0% by weight of SiO₂ was prepared by a similar procedure, as aconventional example of lead borosilicate glass. The glass transitionpoint (Tg) of the thus-obtained glass is shown in the following Table 5.Herein, the glass transition point (Tg) was determined using a thermalanalysis apparatus.

Then, the lead borosilicate-type glass powder was weighed in a givenamount (5.0% by weight), which was followed by milling in theabove-mentioned Ag paste (65% by weight of Ag powder was dissolved into30% by weight of a vehicle in which ethyl cellulose was dissolved intobutyl carbitol) to produce an electrode material for zinc oxidevaristor.

In order to evaluate the electrode material for zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm inthickness) was provided, said sintered-body consisting of bismuth oxide(Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO₂), nickel oxide(NiO), antimony oxide (Sb₂ O₃), and chromium oxide (Cr₂ O₃) respectivelyin 0.5 mole %, and 0.005 mole % of Al₂ _(O) ₃, the rest being zinc oxide(ZnO). On both surfaces of said sintered-body, an electrode material forzinc oxide varistor was applied to be 10 mm in diameter, and then bakedat 800° C. for 10 min. to form electrodes 2. Then, lead wires 3 weresoldered thereon, and thereafter, molded with insulating resin 4 toobtain a sample.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀μA), surge current resistance characteristic and high temperature loadlife performance are shown in the following Table 6. Herein, the abovevoltage ratio (voltage nonlinearity) was obtained through determinationusing a direct current constant current electric source. Further, surgecurrent resistance characteristic was obtained by determining avariation ratio of varistor voltage (V₁ mA) occurring when an impactcurrent of 8/20 μS standard waveform and 5000 A crest value was appliedtwo times in the same direction. Further, high temperature load lifeperformance was obtained by determining a variation ratio of varistorvoltage (V₁ mA) after 1000 hrs. under the conditions of 125° C. ofenvironment temperature and 90% of applied voltage ratio. The number ofsamples was 10 per lot.

                  TABLE 5                                                         ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            MnO.sub.2                                                                           (°C.)                           ______________________________________                                         A*       70       15     15     0     405                                    B         69.9     15     15     0.1   405                                    C         60       15     15     10    430                                    D         45       15     15     25    480                                    E         40       15     15     30    495                                     F*       35       15     15     35    530                                     G*       30       34.9   35     0.1   545                                    H         40       29.9   30     0.1   520                                     I*       89.9     5       5     0.1   315                                     J*       60       0      15     25    460                                    K         55       5      15     25    465                                    L         50       30     15     5     480                                     M*       40       40     15     5     495                                     N*       60       15      0     25    455                                    O         55       15      5     25    465                                    P         50       15     30     5     515                                     Q*       40       15     40     5     525                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 6                                     __________________________________________________________________________                     Surge current resistance                                                                         High temperature load life                                 characteristic ΔV.sub.1 mA (%)                                                             performance ΔV.sub.1 mA (%)         Sample                                                                            Designation  Direction same as                                                                      Direction reverse to                                                                    Direction same as                                                                      Direction reverse to             No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           that of current                                                                        that of current                                                                         that of current                                                                        that of current                  __________________________________________________________________________    1    A*   1.33   -18.4    -27.5     -3.9     -8.8                             2   B     1.13   -14.5    -25.3     +1.3     -3.1                             3   C     1.06   -9.4     -15.5     +1.4     +0.5                             4   D     1.09   -4.3     -7.3      +2.0     +1.6                             5   E     1.12   -12.3    -15.9     +2.2     +1.8                             6    F*   1.24   -20.5    -24.7     +1.2     -2.7                             7    G*   1.10   -22.4    -28.3     +1.1     -2.8                             8   H     1.12   -15.9    -26.4     +1.0     +0.3                             9    I*   1.34   -38.6    -49.7     -5.5     -9.8                             10   J*   1.25   -20.4    -26.0     -1.8     -3.8                             11  K     1.17   -9.2     -16.1     +1.0     +0.2                             12  L     1.10   -10.5    -19.2     +1.8     -0.1                             13   M*   1.13   -22.3    -38.7     +1.7     -1.2                             14   N*   1.12   -21.0    -27.9     +1.3     -3.7                             15  O     1.13   -10.3    -17.1     +1.5     +0.6                             16  P     1.15   -9.8     -18.2     +2.0     +0.7                             17   Q*   1.16   -22.5    -33.4     +1.9     +0.3                             __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 5 and 6 the influence onvoltage nonlinearity by MnO₂ content contained in a leadborosilicate-type glass in an electrode material for a zinc oxidevaristor. The composition systems having MnO₂ content of 0.1% by weightor more are improved in voltage nonlinearity.

Those in which MnO₂ content is more than 30.0% by weight take a bad turnin voltage ratio (voltage nonlinearity) as well as surge currentresistance characteristic. Accordingly, it is a necessary condition thatlead borosilicate-type glass in an electrode material for zinc oxidevaristor is a composition system containing at least 0.1-30.0% by weightof MnO₂.

On the other hand, since surge current resistance characteristic andhigh temperature load life performance are affected by contents of PbO,B₂ O₃ and SiO₂ in addition to Co₃ O₄ content, these compositions arerequired to be considered.

Next, influence on surge current resistance characteristic and hightemperature load life performance by constituents of leadborosilicate-type glass contained in an electrode material for zincoxide varistor will be considered referring to Tables 5 and 6. Glass ofa composition system having PbO content less than 40.0% by weight has ahigher glass transition point Tg and too low a fluidity of glass, whichresult in a deteriorated solder-wetness of glass. Contrarily, glass of acomposition system having PbO content of more than 80.0% by weight has alower glass transition point and too high a fluidity of glass, whichresult in a lower adhesion strength of electrode, and therefore, lacksreliability. In a composition system having B₂ O₃ content of less than5.0% by weight, high temperature load life performance becomes inferior.On the other hand, in a composition system having B₂ O₃ content of morethan 30.0% by weight, surge current resistance characteristic is alsodeteriorated. In a composition system having SiO₂ content of less than5.0% by weight, surge current resistance characteristic is alsodeteriorated. In a composition system having SiO₂ content of more than30.0% by weight, surge current resistance characteristic will alsobecome deteriorated.

From the above results, it is understandable that composition of glasscomponents of electrode material for zinc oxide varistor is optimum tobe in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of MnO₂.

Although lead oxide boron oxide, silicon oxide and manganese oxide wereused, as material of lead borosilicate-type glass, in the forms of PbO,B₂ O₃, SiO₂ and Co₃ O₄, respectively in the present working example, itwas confirmed that the similar characteristics could have also beenobtained by using the other oxide forms. Further, the present workingexample referred only to the case in which lead borosilicate-type glasscontent in electrode material for zinc oxide varistor was 5.0% byweight. However, so far as said content is within 1.0-30.0% by weight,no change is seen in the effect of the present invention. Furthermore,the zinc oxide varistor of a system consisting of ZnO, Bi₂ O₃, Co₃ O₄,MnO₂, NiO, Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ was used as a sintered-body(varistor element 1) for evaluation. However, even when the electrodematerials for a zinc oxide varistor according to the present inventionare applied to a zinc oxide varistor containing Pr₆ O₁₁, CaO, BaO, MgO,K₂ O, SiO₂, etc., no change is seen in effect.

(Working Example 4)

Hereinunder, detailed explanation is made for the 4th working example ofthe present invention.

At first, the description refers to the formulation of glass frit to beincorporated in the electrode material for zinc oxide varistor.According to the composition list of the following Table 7, PbO, B₂ O₃,SiO₂ and Sb₂ O₃ weighed each in a given amount were mixed andsimultaneously ground in a ball mill, and then fused under a temperaturecondition of 1000° C.-1500° C. in a Pt-crucible, which was followed byquenching to be glassified. The thus-obtained glass was roughly crushedand then finely milled in a ball mill to obtain lead borosilicate-typeglass frit. Also, glass powder composed of 70.0% by weight of PbO, 15.0%by weight of B₂ O₃ and 15.0% by weight of SiO₂ was prepared in thesimilar procedure, as a conventional example of lead borosilicate glass.Glass transition point (Tg) the thus-obtained glass was shown in thefollowing Table 7. Herein, glass transition point (Tg) was determinedusing a thermal analysis apparatus.

Then, the lead borosilicate-type glass frit was weighed by 5.0% byweight, which was followed by milling in the above-mentioned Ag paste(65% by weight of Ag powder was dissolved into 30% by weight of avehicle in which ethyl cellulose is dissolved into butyl carbitol) toproduce electrode material for a zinc oxide varistor.

In order to evaluate the electrode material for zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm inthickness) was provided, said sintered-body consisting of bismuth oxide(Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO₂), nickel oxide(NiO), antimony oxide (Sb₂ O₃) and chromium oxide (Cr₂ O₃) respectivelyin 0.5 mole %, and 0.005 mole % of Al₂ O₃, the rest being zinc oxide(ZnO). On both surfaces of said sintered-body, an electrode material forzinc oxide varistor was screen-printed to be 10 mm in diameter, and thenbaked at 800° C. for 10 min. to form electrodes 2. After lead wires 3were soldered thereon, the outer periphery was molded with insulatingresin 4 to obtain a sample.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀μA), limit voltage ratio (V₂₅ A /V₁ mA) and surge current resistancecharacteristics are shown in the following Table 8. The voltage ratioand limit voltage ratio were obtained through determination using adirect current constant current electric source. Further, surge currentresistance characteristic was obtained by determining a variation ratioof varistor voltage (V₁ mA) occurring when an impact current of 8/20 μSstandard waveform and 5000 A crest value was applied two times in thesame direction. The number of samples was 10 per lot.

                  TABLE 7                                                         ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Sb.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*       70       15     15     0     405                                    B         69.9     15     15     0.1   405                                    C         60       15     15     10    435                                    D         45       15     15     25    470                                    E         40       15     15     30    480                                     F*       35       15     15     35    510                                     G*       30       34.9   35     0.1   545                                    H         40       29.9   30     0.1   520                                     I*       89.9     5       5     0.1   315                                     J*       60       0      15     25    450                                    K         55       5      15     25    465                                    L         50       30     15     5     490                                     M*       40       40     15     5     515                                     N*       60       15      0     25    445                                    O         55       15      5     25    455                                    P         50       15     30     5     520                                     Q*       40       15     40     5     535                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 8                                     __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.25 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.16   1.42      -17.5    -25.3                                     3   C     1.09   1.40      -8.4     -14.9                                     4   D     1.07   1.35      -6.3     -9.8                                      5   E     1.13   1.34      -4.6     -7.7                                      6    F*   1.28   1.36      -21.7    -26.4                                     7    G*   1.10   1.53      -22.5    -28.1                                     8   H     1.12   1.46      -10.4    -25.3                                     9    I*   1.34   1.51      -38.9    -49.5                                     10   J*   1.22   1.55      -20.7    -25.1                                     11  K     1.15   1.40      -10.3    -16.8                                     12  L     1.10   1.43      -10.4    -18.7                                     13   M*   1.10   1.50      -22.4    -27.7                                     14   N*   1.08   1.49      -24.1    -27.8                                     15  O     1.11   1.45      -9.5     -16.1                                     16  P     1.15   1.43      -9.8     -15.9                                     17   Q*   1.14   1.48      -21.4    -29.7                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 7 and 8 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by an Sb₂ O₃ contentcontained in a lead borosilicate-type glass frit in an electrodematerial for a zinc oxide varistor. As compared with the leadborosilicate glass of the conventional example containing no Sb₂ O₃, thecomposition systems having an Sb₂ O₃ content of 0.1% by weight or moreare improved in voltage ratio (voltage nonlinearity) but those having anSb₂ O₃ content of more than 30.0% by weight will deteriorate in surgecurrent resistance characteristic. Accordingly, it is a necessarycondition that lead borosilicate-type glass in an electrode material forzinc oxide varistor is a composition system containing at least0.1-30.0% by weight of Sb_(2O) ₃.

On the other hand, since limit voltage ratio characteristic (V₂₅ A /V₁mA) and surge current resistance characteristic are affected by contentsof PbO, B₂ O₃, and SiO₂ in addition to Sb₂ O₃ content, thesecompositions are required to be considered. Therefore, influence onlimit voltage ratio characteristic and surge current resistancecharacteristic and high temperature load life performance byconstituents of lead borosilicate-type glass contained in an electrodematerial for zinc oxide varistor will be considered referring to Tables7 and 8. Glass of a composition system having PbO content less than40.0% by weight has a higher glass transition point (Tg) and too littlea fluidity of glass, which result in a deteriorated solder-wetness ofglass. Contrarily, glass of a composition system having a PbO content ofmore than 80.0% by weight has a lower glass transition point Tg and toohigh a fluidity of glass, which result in a lower adhesion strength ofan electrode. This lacks reliability. In a composition system having aB₂ O₃, content of less than 5.0% by weight, surge current resistancecharacteristic becomes greatly inferior. On the other hand, in acomposition system having a B₂ O₃, content exceeding 30.0% by weight,surge current resistance characteristic is also deteriorated. In acomposition system having a SiO₂ content of less than 5.0% by weight,surge current resistance characteristic is also deteriorated. In acomposition system having SiO₂ content exceeding 30.0% by weight, surgecurrent resistance characteristic will also become deteriorated.

From the above results, it is understandable that composition of glasscomponents of electrode material for zinc oxide varistor is optimum tobe in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of Sb₂ O₃.

Although lead oxide, boron oxide, silicon oxide and antimony oxide wereused, as material of lead borosilicate-type glass, in the forms of PbO,B₂ O₃, SiO₂ and Sb₂ O₃, respectively in the present working example, itwas confirmed that the similar characteristics could have also beenobtained by using the other oxide forms. Further, the present workingexample referred only to the case in which lead borosilicate-type glasscontent in electrode material for a zinc oxide varistor was 5.0% byweight. However, so far as said content is within 1.0-30.0% by weight,no change is seen in the effect of the present invention. Furthermore, azinc oxide varistor of a system consisting of ZnO, Bi₂ O₃, Co₃ O₄, MnO₂,NiO, Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ was used as a sintered-body forevaluation. However, even when the electrode material for zinc oxidevaristor according to the present invention is applied to a zinc oxidevaristor containing Pr₆ O₁₁, CaO, BaO, Sb₂ O₃, K₂ O, SiO₂, etc., nochange is seen in effect.

(Working Example 5)

Hereinunder, detailed explanation is made for the 5th working example ofthe present invention.

At first, the description refers to the formulation of glass frit to beincorporated to electrode material for a zinc oxide varistor. Accordingto the composition list of the following Table 9, PbO, B₂ O₃, SiO₂ andY₂ O₃ each weighed in a given amount were mixed and simultaneouslyground in a ball mill, and then fused under a temperature condition of1000° C.-1500° C. in a Pt-crucible, which was followed by quenching tobe glassified. The thus-obtained glass was roughly crushed and thenfinely milled in a ball mill to obtain lead borosilicate-type glassfrit. Also, glass powder composed of 70.0% by weight of PbO, 15.0% byweight of B₂ O₃, and 15.0% by weight of SiO₂ was prepared by a similarprocedure, as a conventional example of lead borosilicate glass. A glasstransition point (Tg) of the thus-obtained glass is shown in thefollowing Table 9. Herein, glass transition point (Tg) was determinedusing a thermal analysis apparatus.

Then, 5.0% by weight of the lead borosilicate-type glass frit wasweighed, which was followed by milling in the above-mentioned Ag paste(65% by weight of Ag powder was dissolved into 30% by weight of avehicle in which ethyl cellulose is dissolved into butyl carbitol) toproduce electrode material for a zinc oxide varistor.

In order to evaluate the electrode material for zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm inthickness) was provided, said sintered-body consisting of bismuth oxide(Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO₂), nickel oxide(NiO), antimony oxide (Sb₂ O₃) and chromium oxide (Cr₂ O₃) respectivelyin 0.5 mole %, and 0.005 mole % of Al₂ O₃, the rest being zinc oxide(ZnO). On both surfaces of said sintered-body, an electrode material fora zinc oxide varistor was screen-printed to be 10 mm in diameter, andthen baked at 800° C. for 10 min. to form electrodes 2. After lead wires3 were soldered thereon, the outer periphery was with insulative resin 4to obtain a sample.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀μA), limit voltage ratio and surge current resistance characteristic areshown in the following Table 10. The voltage ratio and limit voltageratio were obtained through determination using a direct currentconstant current electric source. Further, surge current resistancecharacteristic was obtained by determining a variation ratio of varistorvoltage (V₁ mA) occurring when an impact current of 8/20 μS standardwaveform and 5000 A crest value was applied two times in the samedirection. The number of samples was 10 per lot.

                  TABLE 9                                                         ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Y.sub.2 O.sub.3                                                                     (°C.)                           ______________________________________                                         A*       70       15     15     0     405                                    B         69.9     15     15     0.1   405                                    C         60       15     15     10    425                                    D         45       15     15     25    470                                    E         40       15     15     30    490                                     F*       35       15     15     35    525                                     G*       30       34.9   35     0.1   545                                    H         40       29.9   30     0.1   520                                     I*       89.9     5       5     0.1   315                                     J*       60       0      15     25    455                                    K         55       5      15     25    465                                    L         50       30     15     5     475                                     M*       40       40     15     5     500                                     N*       60       15      0     25    460                                    O         55       15      5     25    470                                    p         50       15     30     5     510                                     Q*       40       15     40     5     530                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 10                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.25 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.18   1.43      -15.7    -24.4                                     3   C     1.10   1.41      -7.6     -15.3                                     4   D     1.08   1.36      -3.1     -6.2                                      5   E     1.15   1.36      -5.3     -8.8                                      6    F*   1.27   1.39      -15.9    -30.4                                     7    G*   1.15   1.55      -21.3    -31.1                                     8   H     1.18   1.46      -15.3    -24.9                                     9    I*   1.29   1.52      -37.3    -47.5                                     10   J*   1.27   1.53      -17.1    -26.2                                     11  K     1.18   1.45      -10.8    -17.4                                     12  L     1.12   1.42      -10.2    -18.6                                     13   M*   1.11   1.53      -19.7    -28.7                                     14   N*   1.19   1.49      -18.3    -28.2                                     15  O     1.18   1.43      -12.4    -16.9                                     16  P     1.16   1.45      -10.9    -18.3                                     17   Q*   1.19   1.47      -22.1    -31.7                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 9 and 10 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by a Y₂ O₃ content containedin a lead borosilicate-type glass frit in an electrode material for azinc oxide varistor. As compared with the lead borosilicate glass of theconventional example containing no Y₂ O₃, the composition systems havinga Y₂ O₃ content of 0.1% by weight or more are improved in voltage ratio(voltage nonlinearity) but those having a Y₂ O₃ content in excess of30.0% by weight will be deteriorated in surge current resistance.Accordingly, it is a necessary condition that lead borosilicate-typeglass in an electrode material for zinc oxide varistor is a compositionsystem containing at least 0.1-30.0% by weight of Y₂ O₃.

On the other hand, since the limit voltage ratio characteristic (V₂₅ A/V₁ mA) and surge current resistance characteristic are affected bycontents of PbO, B₂ O₃ and SiO₂ in addition a Y₂ O₃ content, thesecompositions are required to be considered. Therefore, influence on thelimit voltage ratio and the surge current resistance characteristic byconstituents of lead borosilicate-type glass contained in an electrodematerial for zinc oxide varistor will be considered on the basis ofTables 9 and 10. Glass of a composition system having a PbO content lessthan 40.0% by weight has a higher glass transition point and too smallfluidity of glass, which result in a deterioration of solder-wetness ofglass. Contrarily, glass of a composition system having PbO content ofmore than 80.0% by weight has a lower glass transition point Tg and toogreat a fluidity of glass, which result in a lower adhesion strength ofan electrode. This lacks reliability. In a composition system having aB₂ O₃ content of less than 5.0% by weight, surge current resistancecharacteristic becomes largely inferior.

On the other hand, in a composition system having a B₂ O₃ content ofmore than 30.0% by weight, surge current resistance characteristic isalso deteriorated. In a composition system having a SiO₂ content of lessthan 5.0% by weight, limit voltage ratio and surge current resistancecharacteristic are also deteriorated. In a composition a system havingSiO₂ content of more than 30.0% by weight, surge current resistancecharacteristic will also become deteriorated.

From the above results is it is understandable that composition of glasscomponents of electrode material for zinc oxide varistor is optimum tobe in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of Y₂ O₃.

Although lead oxide, boron oxide, silicon oxide and antimony oxide wereused, as material of lead borosilicate-type glass, in the forms of PbO,B₂ O₃, SiO₂ and Sb₂ O₃, respectively in the present working example, itwas confirmed that similar characteristics could have also been obtainedby using the other oxide forms. Further, the present working examplerefers only to the case in which a lead borosilicate-type glass contentin an electrode material for a zinc oxide varistor was 5.0% by weight.However, so far as said content is within 1.0-30.0% by weight, no changeis seen in the effect of the present invention. Furthermore, a zincoxide varistor of a system consisting of ZnO, Bi₂ O₃, Co₃ O₄, MnO₂, NiO,Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ was produced into a sintered-body and thenused for evaluation. However, even when the electrode material for azinc oxide varistor according to the present invention is applied to azinc oxide varistor containing Pr₆ O₁₁, CaO, BaO, Sb₂ O₃, K₂ O, SiO₂,etc., no change is seen in effect.

(Working Example 6)

According to the composition list of the following Table 11, PbO, B₂ O₃,SiO₂, Co₂ O₃ and Al₂ O₃ each was weighed in a given amount and thenglass was produced by a procedure similar to that of the above WorkingExample 1, characteristics of the obtained glass are shown in Table 11.

Then, this glass was used to produce an electrode material for a zincoxide varistor as in the above Working Example 1, and further saidmaterial was applied to the zinc oxide varistor element 1 used in theabove Working Example 1to obtain electrode 2.

With respect to the thus-obtained samples, voltage ratio (V₁ mA /V₁₀μA), limit voltage ratio (V₅₀ A /V₁ mA) and surge current resistancecharacteristic are shown in the following Table 12. Herein, the voltageratio and limit voltage ratio were obtained through determination usinga direct current constant current electric source. Further, the surgecurrent resistance characteristic was obtained by determining avariation ratio of varistor voltage (V₁ mA) occurring when an impactcurrent of 8/20 μS standard waveform and 2500 A crest value was appliedtwo times in the same direction. The number of Samples was 10 per lot.

                  TABLE 11                                                        ______________________________________                                        Designation                                                                            Component ratio (wt. %) Tg                                           of glass PbO     B.sub.2 O.sub.3                                                                       SiO.sub.2                                                                          Co.sub.3 O.sub.4                                                                     Al.sub.2 O.sub.3                                                                    (°C.)                       ______________________________________                                        A*       70      15.0    15.0 0      0     405                                B*       69.9    15.0    15.0 0.1    0     405                                C        69.8999 15.0    15.0 0.1    0.0001                                                                              406                                D        59.99   15.0    15.0 10.0   0.01  420                                E*       50.0    15.0    15.0 20.0   0     453                                F        49.9    15.0    15.0 20.0   0.1   455                                G        49.0    15.0    15.0 20.0   1.0   458                                H*       48.5    15.0    15.0 20.0   1.5   463                                I*       40.0    15.0    15.0 30.0   0     475                                J        40.0    14.9    15.0 30.0   0.1   476                                K*       35.0    14.9    15.0 35.0   0.1   488                                L*       30.0    34.9    35.0 0.1    0     545                                M*       30.0    34.8    35.0 0.1    0.1   549                                N*       40.0    29.9    30.0 0.1    0     520                                O        40.0    29.8    30.0 0.1    0.1   526                                P*       84.8    5.0     10.0 0.1    0.1   336                                Q*       64.9    0       15.0 20.0   0.1   437                                R        59.9    5.0     15.0 20.0   0.1   448                                S        49.9    30.0    15.0 5.0    0.1   481                                T        49.0    30.0    15.0 5.0    1.0   485                                U*       44.9    35.0    15.0 5.0    0.1   496                                V*       59.9    15.0    0    25.0   0.1   443                                W        54.9    15.0    5.0  25.0   0.1   445                                X        49.9    15.0    30.0 5.0    0.1   497                                Y        49.0    15.0    3.0  5.0    1.0   506                                Z*       44.9    15.0    35.0 5.0    0.1   510                                ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 12                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________     1  A*    1.83   2.78      -22.3    -28.9                                      2  B*    1.52   2.56      -10.9    -18.0                                      3  C     1.53   2.24      -10.8    -18.3                                      4  D     1.38   1.96      -9.6     -14.4                                      5  E*    1.31   2.48      -4.9     -12.1                                      6  F     1.33   1.86      -5.0     -8.4                                       7  G     1.36   1.87      -9.4     -12.3                                      8  H*    1.42   1.88      -12.6    -15.7                                      9  I*    1.32   2.33      -8.8     -11.9                                     10  J     1.37   2.26      -10.5    -12.5                                     11  K*    1.70   2.24      -20.9    -28.0                                     12  L*    1.51   2.31      -16.2    -23.5                                     13  M*    1.53   2.14      -15.8    -34.6                                     14  N*    1.54   2.12      -12.8    -35.6                                     15  O     1.52   1.95      -10.3    -13.4                                     16  P*    1.73   2.00      -18.2    -32.3                                     17  Q*    1.41   2.21      -20.3    -26.1                                     18  R     1.39   2.19      -10.8    -15.4                                     19  S     1.40   2.31      -9.8     -21.7                                     20  T     1.47   2.25      -11.6    -20.2                                     21  U*    1.43   2.18      -20.3    -22.6                                     22  V*    1.38   2.24      -26.3    -30.1                                     23  W     1.42   1.96      -12.1    -16.8                                     24  X     1.38   2.11      -18.0    -18.0                                     25  Y     1.46   2.02      -11.8    -20.3                                     26  Z*    1.51   2.38      -21.5    -29.6                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 11 and 12 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by Co₃ O₄ and Al₂ O₃contents contained in a lead borosilicate-type glass frit in anelectrode material for a zinc oxide varistor. A composition systemhaving a Co₃ O₄ content of 0.1% by weight or more is improved in voltageratio (voltage nonlinearity) but those having a Co₃ O₄ content of morethan 30.0% by weight will be deteriorated both in voltage ratio (voltagenonlinearity) and surge current resistance. Further, in a compositionsystem having an Al₂ O₃ content of 1.0×10⁻⁴ % by weight or more, limitvoltage ratio characteristic is improved but in a composition systemhaving an Al₂ O₃ content of more than 1.0% by weight, voltage ratio(voltage nonlinearity) and surge current resistance will becomedeteriorated.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for a zinc oxide varistor is a composition systemcontaining 0.1-30.0% by weight of Co₃ O₄ and 1.0×10⁻⁴ -1.0% by weight ofAl₂ O₃.

On the other hand, surge current resistance characteristic and voltageratio (voltage nonlinearity) are affected by contents of PbO, B₂ O₃ andSiO₂ in addition to Co₃ O₄ and Al₂ O₃ contents. However, for similarreasons in the above working examples, it is understandable thatcomposition of glass components of electrode material for zinc oxidevaristor is optimum in a range of 40.0-80.0% by weight of PbO, 5.0-30.0%by weight of B₂ O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weightof Co₃ O₄, in addition to 1.0×10⁻⁴ -1.0% by weight of Al₂ O₃.

Although aluminium oxide (Al₂ _(O) ₃) was used in the present workingexample, it was confirmed that the similar results could have also beenobtained by using at least one of indium oxide (In₂ O₃), gallium oxide(Ga₂ O₃) and germanium oxide (GeO₂) in an amount of 1.0×10⁻⁴ -1.0% byweight, in place of aluminium oxide. Also, it was confirmed that whencombination of these oxides was used, a similar effect could have beenobtained.

(Working Example 7)

According to the composition list of the following Table 13, PbO, B₂ O₃,SiO₂, MgO and Al₂ O₃ were each weighed in a given amount, and then glasswas produced by a procedure similar to that of the above workingexamples. Characteristics of the obtained glass are shown in Table 13.

Then, this glass was used to produce an electrode material for a zincoxide varistor in a similar manner to that of the above workingexamples, and further, said material was applied to the varistor element1 used in the above working example, which was followed by estimation bya similar method. The results are shown in Table 14.

                  TABLE 13                                                        ______________________________________                                        Designation                                                                            Component ratio (wt. %) Tg                                           of glass PbO     B.sub.2 O.sub.3                                                                       SiO.sub.2                                                                          MgO    Al.sub.2 O.sub.3                                                                    (°C.)                       ______________________________________                                        A*       70      15.0    15.0  0     0     405                                B*       69.9    15.0    15.0  0.1   0     405                                C        69.8999 15.0    15.0  0.1   0.0001                                                                              406                                D        59.99   15.0    15.0  10.0  0.01  420                                E*       50.0    15.0    15.0  20.0  0     410                                F        49.9    15.0    15.0  20.0  0.1   416                                G        49.0    15.0    15.0  20.0  1.0   422                                H*       48.5    15.0    15.0  20.0  1.5   430                                I*       40.0    15.0    15.0  30.0  0     420                                J        40.0    14.9    15.0  30.0  0.1   426                                K*       35.0    14.9    15.0  35.0  0.1   445                                L*       30.0    34.9    35.0  0.1   0     545                                M*       30.0    34.8    35.0  0.1   0.1   552                                N*       40.0    29.9    30.0  0.1   0     520                                O        40.0    29.8    30.0  0.1   0.1   526                                P*       84.8    5.0     10.0  0.1   0.1   336                                Q*       64.9    0       15.0  20.0  0.1   405                                R        59.9    5.0     15.0  20.0  0.1   410                                S        49.9    30.0    15.0  5.0   0.1   471                                T        49.0    30.0    15.0  5.0   1.0   480                                U*       44.9    35.0    15.0  5.0   0.1   493                                V*       59.9    15.0    0     25.0  0.1   420                                W        54.9    15.0    5.0   25.0  0.1   435                                X        49.9    15.0    30.0  5.0   0.1   496                                Y        49.0    15.0    30.0  5.0   1.0   502                                Z*       44.9    15.0    35.0  5.0   0.1   506                                ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 14                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________     1  A*    1.83   2.78      -22.3    -28.9                                      2  B*    1.50   2.48      -11.2    -18.3                                      3  C     1.49   2.16      -10.7    -18.8                                      4  D     1.36   1.93      -5.9     -8.7                                       5  E*    1.24   1.88      -5.3     -7.8                                       6  F     1.29   1.80      -4.0     -7.2                                       7  G     1.33   1.86      -8.1     -11.4                                      8  H*    1.41   1.89      -13.2    -16.0                                      9  I*    1.35   2.44      -7.4     -11.7                                     10  J     1.38   2.19      -9.6     -13.2                                     11  K*    1.69   2.32      -19.1    -30.6                                     12  L*    1.51   2.46      -17.8    -24.1                                     13  M*    1.55   2.08      -15.3    -33.7                                     14  N*    1.45   2.49      -11.4    -28.4                                     15  O     1.55   1.92      -10.5    -14.2                                     16  P*    1.71   2.02      -18.0    -27.7                                     17  Q*    1.40   2.30      -13.9    -31.4                                     18  R     1.35   2.13      -11.6    -12.7                                     19  S     1.37   2.24      -12.1    -13.8                                     20  T     1.41   2.20      -12.5    -19.1                                     21  U*    1.43   2.08      -19.4    -28.5                                     22  V*    1.41   2.12      -25.5    -30.6                                     23  W     1.40   1.93      -11.3    -17.3                                     24  X     1.37   2.09      -9.4     -17.7                                     25  Y     1.44   1.97      -10.9    -18.9                                     26  Z*    1.53   2.21      -20.6    -30.1                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 13 and 14 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by MgO and Al₂ O₃ contentscontained in a lead borosilicate-type glass frit in an electrodematerial for a zinc oxide varistor. A composition system having a MgOcontent of 0.1% by weight or more is improved in voltage ratio (voltagenonlinearity) but that having a MgO content of more than 30.0% by weightwill be deteriorated in surge current resistance characteristic.Further, a composition system having an Al₂ O₃ content of 1.0×10⁻⁴ % byweight or more is improved in limit voltage ratio characteristic but acomposition system having an Al₂ O₃ content in excess of 1.0% by weightwill become deteriorated in surge current resistance characteristic.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for zinc oxide varistor is a composition systemcontaining 0.1-30.0% by weight of MgO and 1.0×10⁻⁴ -1.0% by weight ofAl₂ O₃.

On the other hand, surge current resistance characteristic and voltageratio (voltage nonlinearity) are affected by contents of PbO, B₂ O₃ andSiO₂ in addition to MgO and Al₂ O₃ contents. By similar reasons in theabove working examples, it is understandable that composition of glasscomponents of electrode material for a zinc oxide varistor is optimum ina range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂ O₃,5.0-30.0% by weight of SiO₂, 0.1-30.0% by weight of MgO and 1.0×10⁻⁴-1.0% by weight of at least one chemical element selected from Al₂ O₃,In₂ O₃, Ga₂ O₃ and GeO₂.

Aluminium oxide (Al₂ O₃) was used in the present working example, it wasconfirmed that similar results could have also been obtained even whenindium oxide (In₂ O₃), gallium oxide (Ga₂ O₃) and germanium oxide (GeO₂)were used in place of aluminium oxide. Also, it was confirmed that whena combination of these oxides was used, similar results could have beenobtained.

(Working Example 8)

Hereinunder, detailed explanation is made for the 8th working example ofthe present invention.

According to composition list of the following Table 15, PbO, B₂ O₃,SiO₂, Y₂ O₃ and Al₂ O₃ were each weighed each in a given amount, andthen glass was produced by a procedure similar to that of the aboveworking examples. Characteristics of the obtained glass are shown inTable 15.

Then, this glass was used to produce an electrode material for zincoxide varistor in a similar manner to that of the above workingexamples, and further, said material was applied to the varistor element1 used in the above working example to form an electrode, which wasfollowed by evaluation by a similar method. The results are shown inTable 16.

                  TABLE 15                                                        ______________________________________                                        Designation                                                                           Component ratio (wt. %)   Tg                                          of glass                                                                              PbO      B.sub.2 O.sub.3                                                                       SiO.sub.2                                                                           Y.sub.2 O.sub.3                                                                     Al.sub.2 O.sub.3                                                                     (°C.)                      ______________________________________                                        A*      70       15.0    15.0  0     0      405                               B*      69.9     15.0    15.0  0.1   0      405                               C       69.8999  15.0    15.0  0.1   0.0001 406                               D       59.99    15.0    15.0  10.0  0.01   427                               E*      50.0     15.0    15.0  20.0  0      460                               F       49.9     15.0    15.0  20.0  0.1    465                               G       49.0     15.0    15.0  20.0  1.0    467                               H*      48.5     15.0    15.0  20.0  1.5    473                               I       40.0     15.0    15.0  30.0  0      490                               J       40.0     14.9    15.0  30.0  0.1    496                               K*      35.0     14.9    15.0  35.0  0.1    526                               L*      30.0     34.9    35.0  0.1   0      545                               M*      30.0     34.8    35.0  0.1   0.1    544                               N*      40.0     29.9    30.0  0.1   0      520                               O       40.0     29.8    30.0  0.1   0.1    523                               P*      84.8     5.0     10.0  0.1   0.1    330                               Q*      64.9     0       15.0  20.0  0.1    453                               R       59.9     5.0     15.0  20.0  0.1    459                               S       49.9     30.0    15.0  5.0   0.1    478                               T       49.0     30.0    15.0  5.0   1.0    487                               U*      44.9     35.0    15.0  5.0   0.1    493                               V*      59.9     15.0    0     25.0  0.1    463                               W       54.9     15.0    5.0   25.0  0.1    478                               X       49.9     15.0    30.0  5.0   0.1    510                               Y       49.0     15.0    30.0  5.0   1.0    517                               Z*      44.9     15.0    35.0  5.0   0.1    524                               ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 16                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________     1  A*    1.83   2.78     -22.3     -28.9                                      2  B*    1.52   2.57     -10.8     -18.3                                      3  C     1.49   2.32     -11.4     -18.6                                      4  D     1.40   2.01     -8.9      -15.4                                      5  E*    1.33   2.51     -3.8      -7.2                                       6  F     1.36   1.92     -6.7      -7.5                                       7  G     1.40   1.91     -8.9      -13.6                                      8  H*    1.39   1.94     -11.3     -14.2                                      9  I*    1.40   2.38     -9.2      -12.5                                     10  J     1.35   2.22     -11.6     -13.3                                     11  K*    1.66   2.19     -10.3     -27.9                                     12  L*    1.52   2.33     -15.6     -28.3                                     13  M*    1.49   2.17     -15.8     -31.5                                     14  N*    1.53   2.09     -18.2     -34.2                                     15  O     1.48   2.10     -11.3     -12.9                                     16  P*    1.74   2.13     -20.3     -29.8                                     17  Q*    1.43   2.24     -21.1     -26.7                                     18  R     1.40   2.18     -9.3      -11.5                                     19  S     1.41   2.29     -7.8      -18.4                                     20  T     1.46   2.24     -10.3     -19.8                                     21  U*    1.40   2.12     -19.7     -24.3                                     22  V*    1.37   2.30     -25.8     -31.0                                     23  W     1.46   1.82     -11.8     -17.1                                     24  X     1.39   2.16     -10.2     -17.3                                     25  Y     1.45   1.99     -10.9     -19.5                                     26  Z*    1.49   2.33     -20.4     -28.1                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 15 and 16 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by Y₂ O₃ and Al₂ O₃ contentscontained in a lead borosilicate-type glass frit in an electrodematerial for a zinc oxide varistor. A composition system having a Y₂ O₃content of 0.1% by weight or more are improved in voltage ratio (voltagenonlinearity) and surge current resistance characteristic but thathaving a Y₂ O₃ content of more than 30.0% by weight will be deterioratedin both voltage ratio (voltage nonlinearity) as well as surge currentresistance characteristic. Further, a composition system having an Al₂O₃ content of 1.0×10⁻⁴ % by weight or more is improved in limit voltageratio characteristic but a composition system having an Al₂ O₃ contentin excess of 1.0% by weight will become deteriorated in surge currentresistance characteristic.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for zinc oxide varistor is a composition systemcontaining 0.1-30.0% by weight of Y₂ O₃ and 1.0×10⁻⁴ -1.0% by weight ofAl₂ O₃.

On the other hand, surge current resistance characteristic and voltageratio (voltage nonlinearity) are affected by contents of PbO, B₂ O₃ andSiO₂ in addition to the Y₂ O₃ and Al₂ O₃ contents. For similar reasonsin the above working examples, it is understandable that composition ofglass components of electrode material for zinc oxide varistor isoptimum to be in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% byweight of B₂ O₃, 5.0-30.0% by weight of SiO₂, 0.1-30.0% by weight of Y₂O₃ and 1.0×10⁻⁴ -1.0% by weight of at least one chemical elementselected from Al₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

Aluminium oxide (Al₂ O₃) was used in the present working example, but itwas confirmed that the similar results could have also been obtainedeven when indium oxide (In₂ O₃), gallium oxide (Ga₂ O₃) and germaniumoxide (GeO₂) were used in place of aluminium oxide. Also, it wasconfirmed that when a combination of these oxides was used, similarresults could have been obtained.

(Working Example 9)

Hereinunder, detailed explanation is made for the 9th working example ofthe present invention.

According to the composition list of the following Table 17, PbO, B₂ O₃,SiO₂, Sb₂ O₃ and Al₂ O₃ were each weighed in a given amount, and thenglass was produced by the procedure similar to that of the above workingexamples. Characteristics of the obtained glass are shown in Table 17.

Then, this glass was used to produce an electrode material for a zincoxide varistor in a similar manner to that of the above workingexamples, and further, said material was applied to the varistor element1 used in the above working examples to form electrodes 2, which wasfollowed by evaluation in a similar method. The results are shown inTable 18.

                  Table 17                                                        ______________________________________                                        Designation                                                                            Component ratio (wt. %) Tg                                           of glass PbO     B.sub.2 O.sub.3                                                                       SiO.sub.2                                                                          Sb.sub.2 O.sub.3                                                                     Al.sub.2 O.sub.3                                                                    (°C.)                       ______________________________________                                        A*       70      15.0    15.0 0      0     405                                B*       69.9    15.0    15.0 0.1    0     405                                C        69.8999 15.0    15.0 0.1    0.0001                                                                              407                                D        59.99   15.0    15.0 10.0   0.01  438                                E*       50.0    15.0    15.0 20.0   0     460                                F        49.9    15.0    15.0 20.0   0.1   463                                G        49.0    15.0    15.0 20.0   1.0   468                                H*       48.5    15.0    15.0 20.0   1.5   471                                I*       40.0    15.0    15.0 30.0   0     480                                J        40.0    14.9    15.0 30.0   0.1   487                                K*       35.0    14.9    15.0 35.0   0.1   520                                L*       30.0    34.9    35.0 0.1    0     545                                M*       30.0    34.8    35.0 0.1    0.1   550                                N*       40.0    29.9    30.0 0.1    0     520                                O        40.0    29.8    30.0 0.1    0.1   526                                P*       84.8    5.0     10.0 0.1    0.1   339                                Q*       64.9    0       15.0 20.0   0.1   452                                R        59.9    5.0     15.0 20.0   0.1   457                                S        49.9    30.0    15.0 5.0    0.1   498                                T        49.0    30.0    15.0 5.0    1.0   522                                U*       44.9    35.0    15.0 5.0    0.1   535                                V*       59.9    15.0    0    25.0   0.1   451                                W        54.9    15.0    5.0  25.0   0.1   464                                X        49.9    15.0    30.0 5.0    0.1   526                                Y        49.0    15.0    30.0 5.0    1.0   531                                Z*       44.9    15.0    35.0 5.0    0.1   540                                ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 18                                    __________________________________________________________________________                            Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)                  Sample                                                                            Designation         Direction same as                                                                      Direction reverse to                         No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                               that of current                                                                        that of current                              __________________________________________________________________________     1  A*    1.83   2.78   -22.3    -28.9                                         2  B*    1.61   2.52   -11.0    -18.3                                         3  C     1.55   2.36   -10.5    -17.9                                         4  D     1.38   2.12   -9.3     -14.2                                         5  E*    1.35   2.23   -6.8     -9.2                                          6  F     1.36   1.92   -7.7     -8.3                                          7  G     1.39   1.87   -10.9    -12.4                                         8  H*    1.37   1.89   -13.3    -15.2                                         9  I*    1.41   2.34   -9.6     -12.9                                        10  J     1.35   2.15   -10.8    -13.4                                        11  K*    1.45   2.29   -14.3    -29.9                                        12  L*    1.54   2.31   -15.8    -28.5                                        13  M*    1.48   2.18   -16.1    -32.0                                        14  N     1.53   2.16   -17.2    -34.7                                        15  O     1.45   2.13   -12.3    -13.6                                        16  P*    1.69   2.10   -20.7    -30.4                                        17  Q     1.41   2.41   -21.5    -27.1                                        18  R     1.43   2.28   -9.7     -12.0                                        19  S     1.43   2.39   -10.9    -17.4                                        20  T     1.45   2.24   -11.3    -18.7                                        21  U*    1.46   2.31   -20.3    -25.9                                        22  V*    1.40   2.29   -26.7    -32.8                                        23  W     1.45   2.02   -12.8    -16.8                                        24  X     1.42   2.21   -12.1    -17.2                                        25  Y     1.46   1.96   -11.2    -18.3                                        26  Z*    1.47   2.27   -21.4    -27.5                                        __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 17 and 18 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by Sb₂ O₃ and Al₂ O₃contents contained in a lead borosilicate-type glass frit in anelectrode material for a zinc oxide varistor. A composition systemhaving an Sb₂ O₃ content of 0.1% by weight or more is improved involtage ratio (voltage nonlinearity) and surge current resistancecharacteristic but that having a Sb₂ O₃ content of more than 30.0% byweight will be deteriorated in surge current resistance characteristic.Further, a composition system having an Al₂ O₃ content of 1.0×10⁻⁴ % byweight or more is improved in limit voltage ratio characteristic but acomposition system having an Al₂ O₃ content in excess of 1.0% by weightwill become deteriorated in surge current resistance characteristic.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for a zinc oxide varistor is a composition systemcontaining 0.1-30.0% by weight of Sb₂ O₃ and 1.0×10⁻⁴ -1.0% by weight ofAl₂ O₃.

On the other hand, surge current resistance characteristic and voltageratio (voltage nonlinearity) are affected by contents of PbO, B₂ O₃ andSiO₂ in addition to Sb₂ O₃ and Al₂ O₃ contents. For similar reasons asin the above working examples, it is understandable that composition ofglass components of electrode material for a zinc oxide varistor isoptimum in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weightof B₂ O₃, 5.0-30.0% by weight of SiO₂, 0.1-30.0% by weight of Sb₂ O₃ and1.0×10⁻⁴ -1.0% by weight of at least one chemical element selected fromAl₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

Aluminium oxide (Al₂ O₃) was used in the present working example, it wasconfirmed that similar results could also have been obtained even whenindium oxide (In₂ O₃), gallium oxide (Ga₂ O₃) and germanium oxide (GeO₂)were used in place of aluminium oxide. Also, it was confirmed that whena combination of these oxides was used, the similar results could havebeen obtained.

(Working Example 10)

Hereinunder, detailed explanation is made for the 10th working exampleof the present invention.

According to the composition list of the following Table 19, PbO, B₂ O₃,SiO₂, MnO₂ and Al₂ O₃ were each weighed in a given amount, and thenglass was produced by a procedure similar to that of the above workingexamples. Characteristics of the obtained glass are shown in Table 19.

Then, this glass was used to produce an electrode material for zincoxide varistor in a similar manner to that of the above workingexamples, and further, said material was applied to the varistor element1 used in the above working examples to form electrodes 2, which wasfollowed by evaluation by a similar method. The results are shown inTable 20.

                  TABLE 19                                                        ______________________________________                                        Designation                                                                            Component ratio (wt. %) Tg                                           of glass PbO     B.sub.2 O.sub.3                                                                       SiO.sub.2                                                                          MnO.sub.2                                                                            Al.sub.2 O.sub.3                                                                    (°C.)                       ______________________________________                                        A*       70      15.0    15.0 0      0     405                                B*       69.9    15.0    15.0 0.1    0     405                                C        69.8999 15.0    15.0 0.1    0.0001                                                                              405                                D        59.99   15.0    15.0 10.0   0.01  431                                E*       50.0    15.0    15.0 20.0   0     470                                F        49.9    15.0    15.0 20.0   0.1   473                                G        49.0    15.0    15.0 20.0   1.0   480                                H*       48.5    15.0    15.0 20.0   1.5   485                                I*       40.0    15.0    15.0 30.0   0     495                                J        40.0    14.9    15.0 30.0   0.1   502                                K*       35.0    14.9    15.0 35.0   0.1   533                                L*       30.0    34.9    35.0 0.1    0     545                                M*       30.0    34.8    35.0 0.1    0.1   551                                N*       40.0    29.9    30.0 0.1    0     520                                O        40.0    29.8    30.0 0.1    0.1   525                                P*       84.8    5.0     10.0 0.1    0.1   327                                Q*       64.9    0       15.0 20.0   0.1   458                                R        59.9    5.0     15.0 20.0   0.1   466                                S        49.9    30.0    15.0 5.0    0.1   490                                T        49.0    30.0    15.0 5.0    1.0   500                                U*       44.9    35.0    15.0 5.0    0.1   515                                V*       59.9    15.0    0    25.0   0.1   457                                W        54.9    15.0    5.0  25.0   0.1   460                                X        49.9    15.0    30.0 5.0    0.1   519                                Y        49.0    15.0    30.0 5.0    1.0   528                                Z*       44.9    15.0    35.0 5.0    0.1   536                                ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 20                                    __________________________________________________________________________                            Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)                  Sample                                                                            Designation         Direction same as                                                                      Direction reverse to                         No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                               that of current                                                                        that of current                              __________________________________________________________________________     1  A*    1.83   2.78   -22.3   -28.9                                          2  B*    1.53   2.56   -11.1   -17.8                                          3  C     1.49   2.36   -9.9    -12.4                                          4  D     1.38   1.89   -5.1    -8.7                                           5  E*    1.32   2.39   -7.8    -13.6                                          6  F     1.37   1.92   -12.7   -14.9                                          7  G     1.41   1.89   -9.5    -13.0                                          8  H*    1.45   1.91   -12.3   -16.3                                          9  I*    1.39   2.20   -9.7    -12.6                                         10  J     1.44   2.18   -11.6   -13.4                                         11  K*    1.58   2.07   -18.9   -29.2                                         12  L*    1.52   2.29   -16.3   -24.1                                         13  M*    1.49   2.21   -14.9   -35.5                                         14  N*    1.50   2.20   -12.6   -33.1                                         15  O     1.48   1.88   -11.6   -14.2                                         16  P*    1.69   1.93   -16.9   -30.3                                         17  Q*    1.43   2.23   -19.7   -28.9                                         18  R     1.38   2.12   -11.4   -14.7                                         19  S     1.42   2.29   -10.2   -23.1                                         20  T     1.48   2.24   -10.9   -20.5                                         21  U*    1.45   2.33   -21.5   -23.3                                         22  V*    1.39   2.27   -25.8   -31.4                                         23  W     1.40   1.95   -12.3   -15.9                                         24  X     1.39   2.16   -11.7   -17.4                                         25  Y     1.45   1.98   -10.9   -19.1                                         26  Z*    1.50   2.30   -20.8   -30.2                                         __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 19 and 20 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by MnO₂ and Al₂ O₃ contentscontained in a lead borosilicate-type glass frit in an electrodematerial for zinc oxide varistor. A composition system having a MnO₂content of 0.1% by weight or more is improved in voltage ratio (voltagenonlinearity) and surge current resistance characteristic but thathaving a MnO₂ content of more than 30.0% by weight will be deterioratedin both voltage ratio (voltage nonlinearity) and surge currentresistance characteristic. Further, a composition system having an Al₂O₃ content of 1.0×10⁻⁴ % by weight or more is improved in limit voltageratio characteristic but a composition system having an Al₂ O₃ contentin excess of 1.0% by weight will become deteriorated in surge currentresistance characteristic.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for a zinc oxide varistor is a composition systemcontaining 0.1-30.0% by weight of MnO₂ and 1.0×10⁻⁴ -1.0% by weight ofAl₂ O₃.

On the other hand, surge current resistance characteristic and voltageratio (voltage nonlinearity) are affected by contents of PbO, B₂ O₃ andSiO₂ in addition to MnO₂ and Al₂ O₃ contents. For similar reasons in theabove working examples, it is understandable that composition of glasscomponents of electrode material for a zinc oxide varistor is optimum tobe in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight of B₂O₃, 5.0-30.0% by weight of SiO₂, 0.1-30.0% by weight of MnO₂ and1.0×10⁻⁴ -1.0% by weight of at least one chemical element selected fromAl₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

Aluminium oxide (Al₂ O₃) was used in the present working example, it wasconfirmed that the similar results could have also been obtained evenwhen indium oxide (In₂ O₃), gallium oxide (Ga₂ O₃) and germanium oxide(GeO₂) were used in place of aluminium oxide. Also, it was confirmedthat when a combination of these oxides was used, similar results couldhave been obtained.

Further, lead oxide, boron oxide, silicon oxide, manganese oxide,aluminium oxide and indium oxide were used, as material of leadborosilicate-type glass, in the forms of PbO, B₂ O₃, SiO₂, MnO₂, Al₂ O₃and In₂ O₃, respectively in the present working examples 6-10. However,it was confirmed that the similar physical properties could have alsobeen obtained by using the other oxide forms. Further, the presentworking examples 6-10 referred only to the case in which leadborosilicate-type glass content in electrode material for a zinc oxidevaristor was 5.0% by weight, but so far as said content is within1.0-30.0% by weight, no change is seen in the effect of the presentinvention. Furthermore, zinc oxide varistors of systems consisting ofZnO, Bi₂ O₃, Co₂ O₃, MnO₂, NiO, TiO₂, Sb₂ O₃, Cr₂ O₃ and Al₂ O₃ wereused as a sintered-body (varistor element 1) for evaluation. However,even when the electrode material for zinc oxide varistor according tothe present invention is applied to a zinc oxide varistor containing Pr₆O₁₁, CaO, BaO, MgO, K₂ O, SiO₂, etc., no change is seen in effect.

(Working Example 11)

Hereinunder, detailed explanation is made for the 11th working exampleof the present invention.

At first, the description refers to formulation of glass frit to beincorporated to electrode material for a zinc oxide varistor. Accordingto the composition list of the following Table 21, PbO, B₂ O₃, SiO₂ andTeO₂ each weighed in a given amount were mixed and simultaneously groundin a ball mill, and then fused under a temperature condition of 1000°C.-1500° C. in a Pt-crucible, which was followed by quenched to beglassified. The thus-obtained glass was roughly crushed and then finelymilled in a ball mill to obtain lead borosilicate-type glass frit. Also,glass powder composed of 70.0% by weight of PbO, 15.0% by weight of B₂O₃ and 15.0% by weight of SiO₂ was prepared in a similar procedure, as aconventional example of lead borosilicate glass. The glass transitionpoint (Tg) of the thus-obtained glass is shown in the following Table21. Herein, the glass transition point (Tg) was determined using athermal analysis apparatus.

Then, the lead borosilicate-type glass frit was weighed in a givenamount (5.0% by weight), which was followed by milling in theabove-mentioned Ag paste (65% by weight of Ag powder was dissolved into30% by weight of a vehicle, in which ethyl cellulose is dissolved intobutyl carbitol) to produce an electrode material for a zinc oxidevaristor.

In order to evaluate the electrode material for a zinc oxide varistor,which was produced as above, a zinc oxide varistor sintered-body(varistor element 1) (a disk-shape being 13 mm in diameter and 1.5 mm inthickness) was provided, said sintered-body consisting of bismuth oxide(Bi₂ O₃), cobalt oxide (Co₃ O₄), manganese oxide (MnO₂), nickel oxide(NiO), antimony oxide (Sb₂ O₃) and chromium oxide (Cr₂ O₃) respectivelyin 0.5 mole %, and 0.005 mole % of Al₂ O₃, the rest being zinc oxide(ZnO). On both surfaces of said sintered-body, an electrode material forzinc oxide varistor was screen-printed to be 10 mm in diameter, and thenbaked at 750° C. for 10 min. to form electrodes 2, which was followed bysoldering lead wires 3 thereon and subsequently molding with insulativeresin 4 to obtain a sample.

With respect to the thus-obtained samples, voltage ratio (voltagenonlinearity) (V₁ mA /V₁₀ μA), limit voltage ratio characteristic (V₅₀ A/V₁ mA) and, surge current resistance characteristic are shown in thefollowing Table 22. Herein, the voltage ratio (V₁ mA /V₁₀ μA) and limitvoltage ratio (V₅₀ A /V₁ mA) was obtained through determination using adirect current constant current electric source. Further, the surgecurrent resistance characteristic was obtained by determining avariation ratio of varistor voltage (V₁ mA) occurring when an impactcurrent of 8/20 μS standard waveform and 5000 A crest value was appliedtwo times in the same direction. The number of samples was 10 per lot.

                  TABLE 21                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            TeO.sub.2                                                                           (°C.)                           ______________________________________                                        A*        70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   405                                    C         60.0     15.0   15.0   10.0  400                                    D         50.0     15.0   15.0   20.0  405                                    E         40.0     15.0   15.0   30.0  420                                    F*        40.0     10.0   15.0   35.0  425                                    G*        30.0     30.0   30.0   10.0  580                                    H         79.9     10.0   10.0   0.1   360                                    I*        84.9     10.0   5.0    0.1   345                                    J*        70.0     0      20.0   10.0  470                                    K         65.0     5.0    20.0   10.0  485                                    L*        50.0     5.0    35.0   10.0  560                                    M*        70.0     20.0   0      10.0  460                                    N*        50.0     35.0   5.0    10.0  545                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 22                                    __________________________________________________________________________                            Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)                  Sample                                                                            Designation         Direction same as                                                                      Direction reverse to                         No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                               that of current                                                                        that of current                              __________________________________________________________________________    1   A*    1.42   1.67   -18.4   -27.5                                         2   B     1.25   1.53   -16.4   -24.8                                         3   C     1.06   1.48   -4.2    -7.3                                          4   D     1.20   1.47   -5.1    -8.9                                          5   E     1.23   1.47   -7.5    -11.6                                         6   F*    1.35   1.68   -19.3   -26.9                                         7   G*    1.37   1.57   -18.4   -27.1                                         8   H     1.26   1.48   -8.9    -10.2                                         9   I*    1.29   1.51   -12.8   -21.7                                         10  J*    1.36   1.49   -10.3   -18.5                                         11  K     1.22   1.45   -9.7    -18.0                                         12  L*    1.33   1.46   -22.2   -34.5                                         13  M*    1.25   1.47   -17.0   -23.8                                         14  N*    1.22   1.50   -19.6   -41.3                                         __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first there is contemplated from Tables 21 and 22 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by a TeO₂ content containedin a lead borosilicate-type glass in an electrode material for a zincoxide varistor. As shown in Sample No. 6 in Table 22, a compositionsystem having a TeO₂ content of 0.1% by weight or more are improved involtage ratio (voltage nonlinearity) but that having a TeO₂ content ofmore than 30.0% by weight will be deteriorated in limit voltage ratiocharacteristic and surge current resistance characteristic. Accordingly,it is a necessary condition that lead borosilicate-type glass in anelectrode material for zinc oxide varistor is a composition systemcontaining at least 0.1-30.0% by weight of TeO₂.

On the other hand, since surge current resistance characteristic isaffected by contents of PbO, B₂ O₃ and SiO₂ in addition to the TeO₂content, these compositions are required to be considered.

Therefore, influence on limit voltage ratio characteristic and surgecurrent resistance characteristic by constituents of a lead borosilicatetype glass contained in an electrode material will be considered on thebasis of Tables 21 and 22.

Glass of a composition system having PbO content less than 40.0% byweight such as Glass G in Table 21 has a higher glass transition pointTg and too low a fluidity of glass, which result in a deterioratedsolder-wetness of the glass. Contrarily, glass of a composition systemhaving a PbO content in excess of 80.0% by weight, such as Glass I inTable 21 has a lower glass transition point Tg and too great a fluidityof the glass, which result in a lower adhesion strength of electrode.Therefore, this lacks reliability. In a composition system having a B₂O₃ content of less than 5.0% by weight, as shown in Sample No. 10 inTable 22, voltage ratio (voltage nonlinearity) is deteriorated. On theother hand, in a composition system having a B₂ O₃ content in excess of30.0% by weight, as shown in Sample No. 14 in Table 22, surge currentresistance characteristic is also deteriorated. In a composition systemhaving SiO₂ content of less than 5.0% by weight, as shown in Sample No.13 in Table 22, surge current resistance characteristic is alsodeteriorated. In a composition system having a SiO₂ content in excess of30.0% by weight, as shown in Sample No. 12 in Table 22, surge currentresistance characteristic will also become inferior.

From the above results, it is understandable that composition of glasscomponents of an electrode material for a zinc oxide varistor is optimumto be in a range of 40.0-80.0% by weight of PbO, 5.0-30.0% by weight ofB₂ O₃, 5.0-30.0% by weight of SiO₂ and 0.1-30.0% by weight of TeO₂.

(Working Example 12)

Hereinunder, detailed explanation is made for the 12th working exampleof the present invention.

According to the composition list of the following Table 23, PbO, B₂ O₃,SiO₂, TeO₂, Al₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂ were each weighed in a givenamount, and then glass was produced in the similar procedure as in theabove working examples. The characteristics of said glass are shown inTable 23.

Then, this glass was used to produce an electrode material for a zincoxide varistor in a similar manner to those of the above workingexamples. Said material was applied onto the varistor element 1 used inthe above working examples to form electrodes 2. Evaluation was made ina similar manner. The results are shown in Table 24.

                                      TABLE 23                                    __________________________________________________________________________    Designation                                                                         Component ratio (wt. %)       Tg                                        of glass                                                                            PbO B.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        TeO.sub.2                                                                         Al.sub.2 O.sub.3                                                                  In.sub.2 O.sub.3                                                                  Ga.sub.2 O.sub.3                                                                  GeO.sub.2                                                                         (°C.)                              __________________________________________________________________________    C     60.0                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0   0   0   0   400                                       O     59.9999                                                                           15.0                                                                             15.0                                                                             10.0                                                                              0.0001                                                                            0   0   0   400                                       P     59.9                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0   0   0   0   395                                       Q     59.9                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0.05                                                                              0.05                                                                              0   0   395                                       R     59.9                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0   0.1 0   0   390                                       S     59.9                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0   0   0.1 0   400                                       T     59.9                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0   0   0   0.1 395                                        U*   58.5                                                                              15.0                                                                             15.0                                                                             10.0                                                                              1.5 0   0   0   400                                        V*   58.5                                                                              15.0                                                                             15.0                                                                             10.0                                                                              0.05                                                                              0.05                                                                              0.05                                                                              0   395                                       __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 24                                    __________________________________________________________________________                            Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)                  Sample                                                                            Designation         Direction same as                                                                      Direction reverse to                         No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                               that of current                                                                        that of current                              __________________________________________________________________________     3  C     1.06   1.48   -4.2    -7.3                                          15  O     1.06   1.40   -4.0    -7.5                                          16  P     1.07   1.34   -4.5    -8.2                                          17  Q     1.07   1.35   -5.3    -8.7                                          18  R     1.10   1.33   -6.8    -10.0                                         19  S     1.08   1.36   -5.9    -11.8                                         20  T     1.09   1.35   -3.7    -7.1                                          21   U*   1.37   1.38   -16.3   -24.9                                         22   V*   1.41   1.37   -17.2   -30.3                                         __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

At first, there is contemplated from Tables 23 and 24 the influence onvoltage ratio (voltage nonlinearity), limit voltage ratio characteristicand surge current resistance characteristic by Al₂ O₃, In₂ O₃, Ga₂ O₃and GeO₂ contents contained in a lead borosilicate-type glass frit in anelectrode material for zinc oxide varistor. As shown in Sample Nos.15-20 in Table 24, a composition system containing 1.0×10⁻⁴ % by weightof at least one chemical element selected out of Al₂ O₃, In₂ O₃, Ga₂ O₃and GeO₂ is improved in limit voltage ratio characteristic. However, asin Sample Nos. 21 and 22 in Table 24, a composition system in whichamounts to be added of the above chemical elements exceed 1.0% by weightin the total becomes deteriorated in voltage ratio (voltagenonlinearity) and surge current resistance characteristic.

Accordingly, it is a necessary condition that lead borosilicate glass inan electrode material for zinc oxide varistor is a composition systemcontaining 1.0×10⁻⁴ -1.0% by weight of at least one chemical elementselected out of Al₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

On the other hand, surge current resistance characteristic is affectedby contents of PbO, B₂ O₃, SiO₂ and TeO₂ in addition to contents of Al₂O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

For similar reasons in the above working examples, it is understandablethat composition of glass components of electrode material for zincoxide varistor is optimum in a range of 40.0-80.0% by weight of PbO,5.0-30.0% by weight of B₂ O₃, 5.0-30.0% by weight of SiO₂, 0.1-30.0% byweight of TeO₂ and 1.0×10⁻⁴ -1.0% by weight of at least one chemicalelement selected from Al₂ O₃, In₂ O₃, Ga₂ O₃ and GeO₂.

Further, as shown in Sample No. 17 in Table 17, it was confirmed thateven when a combination of the oxides such as Al₂ O₃, In₂ O₃, Ga₂ O₃,GeO₂ and the like, such results as above could have been obtained.

Although lead oxide, boron oxide, silicon oxide tellurium oxide,aluminium oxide and indium oxide were used, as material of leadborosilicate-type glass, in the forms of PbO, B₂ O₃, SiO₂, TeO₂, Al₂ O₃and In₂ O₃, respectively in the present working example, it wasconfirmed that the use of other oxide forms could have also acquiredequal physical properties. Further, the present working example referredonly to the case in which lead borosilicate-type glass content inelectrode material for zinc oxide varistor was 5.0% by weight. However,so far as said content is within 1.0-30.0% by weight, no change is seenin the effect of the present invention. Furthermore, a zinc oxidevaristor of a system consisting of ZnO, Bi₂ O₃, Co₃ O₄, MnO₂, NiO, Sb₂O₃, Cr₂ O₃ and Al₂ O₃ was used as a sintered-body (varistor element 1)for evaluation. However, even when the electrode material for zinc oxidevaristor according to the present invention is applied to a zinc oxidevaristor containing Pr₆ O₁₁, CaO, BaO, MgO, K₂ O, SiO₂, etc., no changeis seen in effect.

Next, a lead borosilicate-type glass containing lanthanoid-series oxideswas fritted in the same manner as in the above working examples. Thisglass frit was milled into the Ag paste same as in the above workingexamples, which was followed by applying onto a fired varistor element 1to form electrodes 2. Hereinunder explanation is given thereon.

The lead borosilicate-type glass in this case contains lanthanoid-seriesoxide (0.1-30.0% by weight), boron oxide (5.0-30.0% by weight), siliconoxide (5.0-30.0% by weight) and lead oxide (40.0-80.0% by weight).

The following Tables 25 and 26 concern those having used lanthanum oxide(LaO₃), in which its content of 0.1% by weight or more will becomebetter in voltage ratio (voltage nonlinearity). Further, when such acontent is more than 30% by weight, glass transition point Tg becomeshigher and the diffusion into varistor element 1 becomes difficult,thereby rendering surge current resistance characteristic to bedeteriorated.

Further, when an amount of boron oxide is less than 5.0% by weight,voltage ratio (voltage nonlinearity) will become inferior, and when itis more than 30%, surge current resistance characteristic will becomedeteriorated.

Furthermore, when silicon oxide content is less than 5.0% by weight,surge current resistance characteristic will become inferior, and whenit is more than 30.0% by weight, voltage ratio (voltage nonlinearity)and surge current resistance characteristic will become deteriorated.

                  TABLE 25                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            La.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*       70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   405                                    C         67.5     15.0   15.0   2.5   415                                    D         65.0     15.0   15.0   5.0   420                                    E         55.0     15.0   20.0   10.0  460                                    F         40.0     10.0   20.0   30.0  518                                     G*       32.5     15.0   20.0   32.5  545                                    H         72.0     3.0    20.0   5.0   415                                    I         70.0     5.0    20.0   5.0   420                                    J         57.5     30.0   10.0   2.5   440                                     K*       52.5     35.0   10.0   2.5   453                                     L*       69.5     25.0   3.0    2.5   420                                    M         72.5     20.0   5.0    2.5   422                                    N         52.5     15.0   30.0   2.5   460                                     O*       50.0     15.0   32.5   2.5   465                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 26                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.20   1.57      -18.0    -25.1                                     3   C     1.08   1.47      -5.1     -10.6                                     4   D     1.06   1.47      -7.3     -12.4                                     5   E     1.07   1.46      -8.9     -17.9                                     6   F     1.10   1.50      -10.4    -22.5                                     7    G*   1.27   1.55      -18.9    -36.2                                     8    H*   1.33   1.50      -15.5    -18.6                                     9   I     1.15   1.52      -11.2    -19.7                                     10  J     1.10   1.50      -10.9    -23.6                                     11   K*   1.11   1.53      -21.4    -32.8                                     12  L     1.15   1.50      -19.8    -38.3                                     13  M     1.17   1.51      -10.7    -23.7                                     14  N     1.22   1.50      -16.6    -24.0                                     15   O*   1.25   1.50      -24.8    -41.6                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

Next, characteristics are shown with respect to the cases having usedtherein the other oxides, in place of lanthanum oxide: cerium oxide inTables 27 and 28, praseodium oxide also in Tables 29 and 30, neodymiumoxide further in Tables 31 and 32, sammarium oxide in Tables 33 and 34,europium oxide in tables 35 and 36, gadolinium oxide in Tables 37 and38, terbium oxide in Tables 39 and 40, dysprosium oxide in Tables 41 and42, holmium oxide in Tables 43 and 44, erbium oxide in Tables 45 and 46,thulium oxide in Tables 47 and 48, yitterbium oxide in Tables 49 and 50,and lutetium oxide in Tables 51 and 52.

In all the above cases, voltage ratio (voltage nonlinearity) becomesbetter, if each lanthanoid-series oxide is contained in an amount of0.1% by weight or more. Further, if it is more than 30% by weight, surgecurrent resistance characteristic will be deteriorated.

                  Table 27                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            CeO.sub.2                                                                           (°C.)                           ______________________________________                                         A*       70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   405                                    C         67.5     15.0   15.0   2.5   415                                    D         65.0     15.0   15.0   5.0   420                                    E         55.0     15.0   20.0   10.0  465                                    F         40.0     10.0   20.0   30.0  515                                     G*       32.5     15.0   20.0   32.5  540                                     H*       72.0     3.0    20.0   5.0   412                                    I         70.0     5.0    20.0   5.0   417                                    J         57.5     30.0   10.0   2.5   435                                     K*       52.5     35.0   10.0   2.5   455                                     L*       69.5     25.0   3.0    2.5   420                                    M         72.5     20.0   5.0    2.5   425                                    N         52.5     15.0   30.0   2.5   460                                     O*       50.0     15.0   32.5   2.5   467                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 28                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.21   1.56      -17.9    -24.8                                     3   C     1.08   1.46      -4.8     -9.2                                      4   D     1.05   1.47      -6.9     -11.0                                     5   E     1.08   1.47      -8.8     -17.4                                     6   F     1.11   1.49      -9.7     -21.7                                     7    G*   1.27   1.53      -20.3    -36.0                                     8    H*   1.32   1.50      -14.8    -20.7                                     9   I     1.14   1.52      -11.3    -18.5                                     10  J     1.11   1.50      -10.4    -21.1                                     11   K*   1.10   1.51      -19.7    -32.6                                     12   L*   1.16   1.50      -19.3    -36.3                                     13  M     1.17   1.50      -10.9    -20.8                                     14  N     1.23   1.51      -15.1    -21.3                                     15   O*   1.25   1.49      -25.1    -42.1                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 29                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Pr.sub.6 O.sub.11                                                                   (°C.)                           ______________________________________                                         A*       70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   405                                    C         67.5     15.0   15.0   2.5   417                                    D         65.0     15.0   15.0   5.0   422                                    E         55.0     15.0   20.0   10.0  460                                    F         40.0     10.0   20.0   30.0  515                                     G*       32.5     15.0   20.0   32.5  547                                     H*       72.0     3.0    20.0   5.0   420                                    I         70.0     5.0    20.0   5.0   418                                    J         57.5     30.0   10.0   2.5   440                                     K*       52.5     35.0   10.0   2.5   445                                     L*       69.5     25.0   3.0    2.5   425                                    M         72.5     20.0   5.0    2.5   427                                    N         52.5     15.0   30.0   2.5   460                                     O*       50.0     15.0   32.5   2.5   465                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 30                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.22   1.59      -18.0    -26.2                                     3   C     1.09   1.47      -5.6     -10.8                                     4   D     1.07   1.46      -7.8     -12.7                                     5   E     1.10   1.46      -9.5     -18.5                                     6   F     1.12   1.48      -11.2    -21.9                                     7    G*   1.26   1.51      -20.4    -37.0                                     8    H*   1.35   1.49      -16.8    -19.2                                     9   I     1.16   1.50      -11.3    -20.2                                     10  J     1.12   1.50      -11.0    -24.8                                     11   K*   1.11   1.52      -21.1    -33.1                                     12   L*   1.15   1.51      -19.6    -40.3                                     13  M     1.16   1.50      -11.0    -24.9                                     14  N     1.23   1.50      -16.2    -22.6                                     15   O*   1.28   1.51      -25.3    -42.8                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 31                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Nd.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*       70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   406                                    C         67.5     15.0   15.0   2.5   417                                    D         65.0     15.0   15.0   5.0   420                                    E         55.0     15.0   20.0   10.0  470                                    F         40.0     10.0   20.0   30.0  520                                     G*       32.5     15.0   20.0   32.5  550                                     H*       72.0     3.0    20.0   5.0   420                                    I         70.0     5.0    20.0   5.0   415                                    J         57.5     30.0   10.0   2.5   440                                     K*       52.5     35.0   10.0   2.5   457                                     L*       69.5     25.0   3.0    2.5   423                                    M         72.5     20.0   5.0    2.5   430                                    N         52.5     15.0   30.0   2.5   465                                     O*       50.0     15.0   32.5   2.5   470                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 32                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.19   1.55      -18.1    -26.4                                     3   C     1.08   1.46      -6.3     -11.2                                     4   D     1.06   1.47      -8.0     -12.9                                     5   E     1.06   1.46      -10.7    -17.1                                     6   F     1.08   1.50      -12.4    -21.6                                     7    G*   1.29   1.53      -20.3    -37.3                                     8    H*   1.31   1.50      -16.3    -19.2                                     9   I     1.16   1.51      -11.4    -19.4                                     10  J     1.10   1.50      -11.8    -23.0                                     11   K*   1.12   1.53      -20.4    -33.7                                     12   L*   1.14   1.49      -19.8    -38.5                                     13  M     1.17   1.50      -11.2    -22.9                                     14  N     1.23   1.50      -15.3    -23.8                                     15   O*   1.26   1.50      -25.0    -42.4                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 33                                                        ______________________________________                                        Designation                                                                             Component ratio (wt. %)                                                                            Tg                                             of glass  PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                            Sm.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*       70.0     15.0   15.0   0     405                                    B         69.9     15.0   15.0   0.1   405                                    C         67.5     15.0   15.0   2.5   415                                    D         65.0     15.0   15.0   5.0   422                                    E         55.0     15.0   20.0   10.0  465                                    F         40.0     10.0   20.0   30.0  525                                     G*       32.5     15.0   20.0   32.5  553                                     H*       72.0     3.0    20.0   5.0   413                                    I         70.0     5.0    20.0   5.0   415                                    J         57.5     30.0   10.0   2.5   442                                     K*       52.5     35.0   10.0   2.5   458                                     L*       69.5     25.0   3.0    2.5   425                                    M         72.5     20.0   5.0    2.5   430                                    N         52.5     15.0   30.0   2.5   460                                     O*       50.0     15.0   32.5   2.5   465                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                                      TABLE 34                                    __________________________________________________________________________                               Surge current resistance                                                      characteristic ΔV.sub.1 mA (%)               Sample                                                                            Designation  Limit voltage ratio                                                                     Direction same as                                                                      Direction reverse to                      No. of glass                                                                            V.sub.1 mA /V.sub.10 μA                                                           V.sub.50 A /V.sub.1 mA                                                                  that of current                                                                        that of current                           __________________________________________________________________________    1    A*   1.33   1.57      -18.4    -27.5                                     2   B     1.20   1.56      -17.9    -26.1                                     3   C     1.07   1.47      -5.9     -11.3                                     4   D     1.05   1.48      -9.4     -13.1                                     5   E     1.07   1.47      -9.8     -17.8                                     6   F     1.09   1.50      -12.6    -22.0                                     7    G*   1.28   1.54      -21.0    -38.5                                     8    H*   1.33   1.50      -17.5    -19.9                                     9   I     1.15   1.52      -10.6    -20.8                                     10  J     1.09   1.50      -11.9    -25.2                                     11   K*   1.13   1.53      -22.2    -32.3                                     12   L*   1.15   1.50      -20.2    -41.8                                     13  M     1.15   1.50      -11.1    -23.9                                     14  N     1.22   1.51      -16.4    -21.8                                     15   O*   1.25   1.49      -25.6    -42.6                                     __________________________________________________________________________     *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 35                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Eu.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   407                                    C        55.0     15.0   20.0    10.0  470                                    D        40.0     10.0   20.0    30.0  523                                     E*      32.5     15.0   20.0    32.5  550                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 36                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.21    1.57     -18.0   -26.5                                  3    C        1.08    1.47     -9.7    -18.2                                  4    D        1.10    1.49     -11.9   -21.8                                  5     E*      1.30    1.52     -20.3   -39.7                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 37                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Gd.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   407                                    C        55.0     15.0   20.0    10.0  470                                    D        40.0     10.0   20.0    30.0  523                                     E*      32.5     15.0   20.0    32.5  550                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 38                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.22    1.56     -17.9   -26.1                                  3    C        1.08    1.47     -9.3    -18.7                                  4    D        1.10    1.48     -12.2   -22.0                                  5     E*      1.30    1.51     -20.8   -39.5                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 39                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Tb.sub.4 O.sub.7                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   405                                    C        55.0     15.0   20.0    10.0  475                                    D        40.0     10.0   20.0    30.0  520                                     E*      32.5     15.0   20.0    32.5  550                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 40                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.20    1.55     -18.1   -26.3                                  3    C        1.09    1.48     -9.9    -19.1                                  4    D        1.09    1.49     -12.0   -22.6                                  5     E*      1.31    1.50     -21.1   -40.4                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 41                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Dy.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   405                                    C        55.0     15.0   20.0    10.0  472                                    D        40.0     10.0   20.0    30.0  528                                     E*      32.5     15.0   20.0    32.5  555                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 42                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.22    1.57     -17.8   -26.1                                  3    C        1.09    1.48     -9.2    -19.3                                  4    D        1.10    1.49     -11.8   -22.5                                  5     E*      1.31    1.50     -20.7   -39.6                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 43                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Ho.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   407                                    C        55.0     15.0   20.0    10.0  475                                    D        40.0     10.0   20.0    30.0  532                                     E*      32.5     10.0   25.0    32.5  560                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 44                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.22    1.57     -18.1   -25.4                                  3    C        1.09    1.47     -10.3   -19.7                                  4    D        1.10    1.48     -11.7   -22.9                                  5     E*      1.31    1.51     -19.2   -39.8                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 45                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Er.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   408                                    C        55.0     15.0   20.0    10.0  477                                    D        40.0     10.0   20.0    30.0  530                                     E*      32.5     10.0   25.0    32.5  558                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 46                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.24    1.56     -18.0   -25.7                                  3    C        1.10    1.50     -11.2   -19.3                                  4    D        1.15    1.50     -11.8   -22.4                                  5     E*      1.35    1.52     -21.6   -40.6                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 47                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Tm.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   405                                    C        55.0     15.0   20.0    10.0  475                                    D        40.0     10.0   20.0    30.0  535                                     E*      32.5     10.0   25.0    32.5  565                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 48                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.25    1.55     -18.0   -26.4                                  3    C        1.10    1.49     -9.3    -20.2                                  4    D        1.13    1.48     -12.8   -23.5                                  5     E*      1.33    1.51     -21.5   -41.1                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 49                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Yb.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   405                                    C        55.0     15.0   20.0    10.0  475                                    D        40.0     10.0   20.0    30.0  530                                     E*      32.5     10.0   25.0    32.5  558                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 50                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.24    1.56     -18.2   -27.1                                  3    C        1.11    1.50     -10.4   -19.8                                  4    D        1.12    1.48     -13.0   -24.1                                  5     E*      1.36    1.53     -21.6   -42.5                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 51                                                        ______________________________________                                        Designa-                                                                      tion of  Component ratio (wt. %)                                                                             Tg                                             glass    PbO      B.sub.2 O.sub.3                                                                      SiO.sub.2                                                                             Lu.sub.2 O.sub.3                                                                    (°C.)                           ______________________________________                                         A*      70.0     15.0   15.0    0     405                                    B        69.9     15.0   15.0    0.1   407                                    C        55.0     15.0   20.0    10.0  480                                    D        40.0     10.0   20.0    30.0  540                                     E*      32.5     10.0   25.0    32.5  565                                    ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

                  TABLE 52                                                        ______________________________________                                                           Surge current resistance                                                      characteristic                                                                ΔV.sub.1 mA (%)                                           Desig-           Limit    Direction                                                                             Direction                              Sam- nation           voltage  same as reverse                                ple  of       V.sub.1 mA /                                                                          ratio    that of to that                                No.  glass    V.sub.10 μA                                                                        V.sub.50 A /V.sub.1 mA                                                                 current of current                             ______________________________________                                        1     A*      1.33    1.57     -18.4   -27.5                                  2    B        1.25    1.55     -18.2   -26.8                                  3    C        1.12    1.51     -10.3   -15.9                                  4    D        1.14    1.50     -13.7   -23.8                                  5     E*      1.36    1.51     -21.0   -43.5                                  ______________________________________                                         *are comparative examination examples which are outside of the present        invention.                                                               

The above working examples indicated the cases in which a leadborosilicate glass frit is milled into Ag-paste and then applied ontovaristor element 1 to form electrodes 2, and upon baking of electrodes2, chemical elements constituting said lead borosilicate glass frit arediffused into the varistor element 1. However, the present invention isnot limited to said procedure. A similar effect concerning voltage ratio(voltage nonlinearity) has been obtained also by the followingprocedure, wherein prior to the formation of electrodes 2, a pastecontaining a lead borosilicate-type glass frit is applied onto a surfaceof a fired varistor element 1 and then the resultant is heated undersuch a state as it is, thereby allowing the chemical elements composingsaid lead borosilicate-type glass frit to penetrate into varistorelement 1, and thereafter, a Ag-paste containing no leadborosilicate-type glass frit is used to form electrodes 2.

Further, an electrode material for forming electrodes 2 is not limitedto Ag-paste, which may be replaced with pastes of the other metals suchas Pd, etc.

INDUSTRIALLY AVAILABLE FIELD

As mentioned above, according to the present invention, there isdiffused from a surface of a fired varistor element a leadborosilicate-type glass containing at least one metal oxide selected outof cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide.

Thus, when voltage nonlinearity is so improved, energy saving andefficiency improvement can be seen for various kinds of electronicinstruments to be used owing to these being less leakage current.

We claim:
 1. A zinc oxide varistor comprising a fired varistor elementhaving opposite surfaces and at least two electrodes formed on saidfired varistor element from an electrode paste, said fired varistorelement comprising a lead borosilicate-type glass diffused into at leastone of said surfaces of said fired varistor element during a heatingoperation employed to form said electrodes; said lead borosilicate-typeglass comprising a mixture of lead borosilicate-type glass particulatematerial and at least one metal oxide selected from the group consistingof cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide;wherein theglass particulate material and the at least one metal oxide are mixed toform said mixture, and then said mixture is fused and thereafterquenched; with the provisos that if the at least one metal oxidecomprises at least one member of the group consisting of cobalt oxideand manganese oxide, upon mixing the glass particulate material and theat least one metal oxide to form said mixture, said mixture contains5.0-30% by weight of boron oxide, 5.0-30% by weight of silicon oxide,40.0-80% by weight of lead oxide, and 0.1%-30.0% by weight of said atleast one metal oxide.
 2. The zinc oxide varistor of claim 1, whereinthe lead borosilicate type glass is diffused from said electrode pastethrough the surface of said fired varistor element, into said firedvaristor element.
 3. The zinc oxide varistor according to claim 1,wherein said mixture contains 0.1-30% by weight cobalt oxide.
 4. Thezinc oxide varistor according to claim 1, wherein said mixture contains0.1-30% by weight manganese oxide.
 5. The zinc oxide varistor accordingto claim 1, wherein the glass particulate material and the at least onemetal oxide are mixed to form said mixture, and then the mixture isfused and thereafter quenched, said mixture, upon forming, contains5.0-30% by weight of boron oxide, 5.0-30% by weight of silicon oxide,40.0-80% by weight of lead oxide and 0.1%-30.0% by weight of said atleast one metal oxide.
 6. The zinc oxide varistor according to claim 5,wherein said mixture contains 0.1-30% by weight magnesium oxide.
 7. Thezinc oxide varistor according to claim 5, wherein said mixture contains0.1-30% by weight yttrium oxide.
 8. The zinc oxide varistor according toclaim 5, wherein said mixture contains 0.1-30% by weight antimony oxide.9. The zinc oxide varistor according to claim 5, wherein said mixturecontains 0.1-30% by weight tellurium oxide.
 10. The zinc oxide varistoraccording to claim 5, wherein said mixture contains 0.1-30% by weightlanthanum oxide.
 11. The zinc oxide varistor according to claim 5,wherein said mixture contains 0.1-30% by weight cerium oxide.
 12. Thezinc oxide varistor according to claim 5, wherein said mixture contains0.1-30% by weight praseodium oxide.
 13. The zinc oxide varistoraccording to claim 5, wherein said mixture contains 0.1-30% by weightneodymium oxide.
 14. The zinc oxide varistor according to claim 5,wherein said mixture contains 0.1-30% by weight samarium oxide.
 15. Thezinc oxide varistor according to claim 5, wherein said mixture contains0.1-30% by weight europium oxide.
 16. The zinc oxide varistor accordingto claim 5, wherein said mixture contains 0.1-30% by weight gandolinumoxide.
 17. The zinc oxide varistor according to claim 5, wherein saidmixture contains 0.1-30% by weight terbium oxide.
 18. The zinc oxidevaristor according to claim 5, wherein said mixture contains 0.1-30% byweight dysprosium oxide.
 19. The zinc oxide varistor according to claim5, wherein said mixture contains 0.1-30% by weight holmium oxide. 20.The zinc oxide varistor according to claim 5, wherein said mixturecontains 0.1-30% by weight erbium oxide.
 21. The zinc oxide varistoraccording to claim 5, wherein said mixture contains 0.1-30% by weightthulium oxide.
 22. The zinc oxide varistor according to claim 5, whereinsaid mixture contains 0.1-30% by weight ytterbium oxide.
 23. The zincoxide varistor according to claim 5, wherein said mixture contains0.1-30% by weight lutetium oxide.
 24. A zinc oxide varistor comprising afired varistor element having opposite surfaces, and at least twoelectrodes formed on said fired varistor element from an electrodepaste, said fired varistor element comprising a lead borosilicate-typeglass diffused into at least one surface of said fired varistor elementduring a heating operation employed to form said electrodes; said leadborosilicate-type glass comprising a mixture of lead borosilicate-typeglass particulate material and at least one first metal oxide selectedfrom the group consisting of cobalt oxide, magnesium oxide, yttriumoxide, antimony oxide, manganese oxide, tellurium oxide, lanthanumoxide, cerium oxide, praseodium oxide, neodymium oxide, samarium oxide,europium oxide, gandolinium oxide, terbium oxide, dysprosium oxide,holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetiumoxide, and at least one second metal oxide of aluminum oxide, indiumoxide, germanium oxide and gallium oxide.
 25. The zinc oxide varistoraccording to claim 24, wherein said at least one second metal oxide ispresent in said mixture in an amount of 1.0×10⁻⁴ -1.0% by weight of saidmixture.
 26. The zinc oxide varistor according to claim 24, with theproviso that if the at least one metal oxide comprises at least onemember of the group consisting of cobalt oxide and manganese oxide, theglass particulate material and the at least one metal oxide are mixed toform said mixture and, upon forming, said mixture contains 5-30% byweight of boron oxide, 5-30% by weight of silicon oxide, 40.0-80% byweight of lead oxide, and 0.1-30.0% by weight of said at least one metaloxide.
 27. The zinc oxide varistor according to claim 24, wherein thelead borosilicate-type glass particulate material and the at least onemetal oxide are mixed to form said mixture, and then the mixture isfused and thereafter quenched, said mixture, upon forming, contains5.0-30% by weight of boron oxide, 5.0-30% by weight of silicon oxide,40.0-80% by weight of lead oxide and 0.1%-30.0% by weight of said atleast one metal oxide.
 28. The zinc oxide varistor according to claim27, wherein the mixture contains 0.1-30.0% by weight cobalt oxide. 29.The zinc oxide varistor according to claim 27, wherein the mixturecontains 0.1-30.0% by weight magnesium oxide.
 30. The zinc oxidevaristor according to claim 27, wherein the mixture contains 0.1-30.0%by weight yttrium oxide.
 31. The zinc oxide varistor according to claim27, wherein the mixture contains 0.1-30.0% by weight antimony oxide. 32.The zinc oxide varistor according to claim 27, wherein the mixturecontains 0.1-30.0% by weight manganese oxide.
 33. The zinc oxidevaristor according to claim 27, wherein the mixture contains 0.1-30.0%by weight tellurium oxide.
 34. The zinc oxide varistor according toclaim 27, wherein the mixture contains 0.1-30.0% by weight lanthanumoxide.
 35. The zinc oxide varistor according to claim 27, wherein themixture contains 0.1-30.0% by weight cerium oxide.
 36. The zinc oxidevaristor according to claim 27, wherein the mixture contains 0.1-30.0%by weight praseodymium oxide.
 37. The zinc oxide varistor according toclaim 27, wherein the mixture contains 0.1-30.0% by weight neodymiumoxide.
 38. The zinc oxide varistor according to claim 27, wherein themixture contains 0.1-30.0% by weight samarium oxide.
 39. The zinc oxidevaristor according to claim 27, wherein the mixture contains 0.1-30.0%by weight europium oxide.
 40. The zinc oxide varistor according to claim27, wherein the mixture contains 0.1-30.0% by weight gadolinium oxide.41. The zinc oxide varistor according to claim 27, wherein the mixturecontains 0.1-30.0% by weight terbium oxide.
 42. The zinc oxide varistoraccording to claim 27, wherein the mixture contains 0.1-30.0% by weightdysprosium oxide.
 43. The zinc oxide varistor according to claim 27,wherein the mixture contains 0.1-30.0% by weight holmium oxide.
 44. Thezinc oxide varistor according to claim 27, wherein the mixture contains0.1-30.0% by weight erbium oxide.
 45. The zinc oxide varistor accordingto claim 27, wherein the mixture contains 0.1-30.0% by weight thuliumoxide.
 46. The zinc oxide varistor according to claim 27, wherein themixture contains 0.1-30.0% by weight ytterbium oxide.
 47. The zinc oxidevaristor according to claim 27, wherein the mixture contains 0.1-30.0%by weight lutetium oxide.
 48. A process for producing a zinc oxidevaristor characterized by diffusing a lead borosilicate-type glass intoa surface of a fired varistor element, and providing said varistorelement with at least two electrodes, said lead borosilicate-type glasscomprising a mixture of lead borosilicate-type glass particulatematerial and at least one metal oxide selected from the group consistingof cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide, wherein theglass particulate material and the at least one metal oxide are mixed toform said mixture and then said mixture is fused and thereafterquenched,with the provisos that if the glass comprises at least onemember of the group consisting of cobalt oxide and manganese oxide, thenupon mixing the glass particulate material and the at least one metaloxide to form said mixture, said mixture contains 5.0-30% by weight ofboron oxide, 5.0-30% by weight of silicon oxide, 40.0-80% by weight oflead oxide, and 0.1%-30.0% by weight of said at least one metal oxide.49. The process for producing a zinc oxide varistor according to claim48, wherein the lead borosilicate-type glass particulate material andthe at least one metal oxide are mixed to form said mixture, and thenthe mixture is fused and thereafter quenched, said mixture, uponforming, contains 5.0-30% by weight of boron oxide, 5.0-30% by weight ofsilicon oxide, 40.0-80% by weight of lead oxide and 0.1%-30.0% by weightof said at least one metal oxide.
 50. A process for producing a zincoxide varistor comprising diffusing a lead borosilicate-type glass intoa surface of a fired varistor element, and providing said varistorelement with at least two electrodes, said lead borosilicate-type glasscomprising a mixture of lead borosilicate-type glass particulatematerial and at least one metal oxide selected from the group consistingof cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide, which ischaracterized by applying said lead borosilicate-type glass onto saidsurface of said fired varistor element, and then heating it, therebyhaving said lead borosilicate-type glass diffuse from said surface ofthe fired varistor element into the fired varistor element.
 51. Theprocess according to claim 50, with the proviso that if the at least onemetal oxide comprises at least one member of the group consisting ofcobalt oxide and manganese oxide, the glass particulate material and theat least one metal oxide are mixed to form said mixture and, uponforming, said mixture contains 5-30% by weight of boron oxide, 5-30% byweight of silicon oxide, 40.0-80% by weight of lead oxide, and 0.1-30.0%by weight of said at least one metal oxide.
 52. A process for producinga zinc oxide varistor comprising diffusing a lead borosilicate-typeglass into a surface of a fired varistor element, and providing saidfired varistor element with at least two electrodes, said leadborosilicate-type glass comprising a mixture of lead borosilicate-typeglass particulate material at least one metal oxide selected from thegroup consisting of cobalt oxide, magnesium oxide, yttrium oxide,antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide,cerium oxide, praseodium oxide, neodymium oxide, samarium oxide,europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide,holmium oxide, erbium oxide, thulium oxide, ytterbium oxide and lutetiumoxide, and at least one member of the group consisting of aluminum,indium, gallium and germanium.
 53. The process according to claim 52,with the proviso that if the at least one metal oxide comprises at leastone member of the group consisting of cobalt oxide and manganese oxide,the glass particulate material and the at least one metal oxide aremixed to form said mixture and, upon forming, said mixture contains5-30% by weight of boron oxide, 5-30% by weight of silicon oxide,40.0-80% by weight of lead oxide, and 0.1-30.0% by weight of said atleast one metal oxide.
 54. A process for producing a zinc oxide varistorcomprising diffusing a lead borosilicate-type glass into a surface of afired varistor element, and providing said fired varistor element withat least two electrodes, said lead borosilicate-type glass comprising amixture of lead borosilicate-type glass,particulate material at leastone metal oxide selected from the group consisting of cobalt oxide,magnesium oxide, yttrium oxide, antimony oxide, manganese oxide,tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide,neodymium oxide, samarium oxide, europium oxide, gadolinium oxide,terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thuliumoxide, ytterbium oxide and lutetium oxide, and at least one member ofthe group consisting of aluminum oxide, indium oxide, gallium oxide andgermanium oxide.
 55. A process for producing a zinc oxide varistorcomprising diffusing a lead borosilicate-type glass into a surface of afired varistor element, and providing said fired varistor element withat least two electrodes, said lead borosilicate-type glass comprising amixture of lead borosilicate-type glass particulate material and atleast one metal oxide selected from the group consisting of cobaltoxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide,tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide,neodymium oxide, samarium oxide, europium oxide, gadolinium oxide,terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thuliumoxide, ytterbium oxide and lutetium oxide, which is characterized byapplying said lead borosilicate-type glass onto a surface of saidvaristor, and then adding at least one of aluminum, indium, gallium andgermanium onto a surface of said lead borosilicate-type glass.
 56. Aprocess for producing a zinc oxide varistor comprising diffusing a leadborosilicate-type glass into a surface of a fired varistor element, andproviding said fired varistor element with at least two electrodes, saidlead borosilicate-type glass comprising a mixture of leadborosilicate-type glass particulate material and at least one metaloxide selected from the group consisting of cobalt oxide, magnesiumoxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide,lanthanum oxide, cerium oxide, praseodium oxide, neodymium oxide,samarium oxide, europium oxide, gadolinium oxide, terbium oxide,dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbiumoxide and lutetium oxide, which is characterized by applying said leadborosilicate-type glass onto a surface of said varistor element, andthen adding at least one of aluminum oxide, indium oxide, gallium oxideand germanium oxide onto a surface of said lead borosilicate-type glass.57. A process for producing a zinc oxide varistor comprising adding alead borosilicate-type glass to an electrode paste, and then applyingthe resulting electrode paste onto a surface of a fired varistorelement, which is followed by baking the fired varistor element to forman electrode from the electrode paste, said lead borosilicate-type glasscomprising a mixture of lead borosilicate-type glass particulatematerial and at least one metal oxide selected from the group consistingof cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide,manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide,praseodium oxide, neodymium oxide, samarium oxide, europium oxide,gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbiumoxide, thulium oxide, ytterbium oxide and lutetium oxide, saidborosilicate-type glass being diffused from the electrode paste toinside the fired varistor element.
 58. The process for producing a zincoxide varistor according to claim 57, further comprising adding at leastone chemical element of aluminium, indium, gallium and germanium, intothe electrode paste which contains a lead borosilicate-type glass. 59.The process for producing a zinc oxide varistor according to claim 57,further comprising adding at least one of aluminium oxide, indium oxide,gallium oxide and germanium oxide into the electrode paste.
 60. Theprocess according to claim 57, with the proviso that if the at least onemetal oxide comprises at least one member of the group consisting ofcobalt oxide and manganese oxide, the glass particulate material and theat least one metal oxide are mixed to form said mixture and, uponforming, said mixture contains 5-30% by weight of boron oxide, 5-30% byweight of silicon oxide, 40.0-80% by weight of lead oxide, and 0.1-30.0%by weight of said at least one metal oxide.
 61. A process for producinga zinc oxide varistor comprising diffusing a lead borosilicate-typeglass into a surface of a fired varistor element, and providing saidvaristor element with at least two electrodes, said leadborosilicate-type glass comprising a mixture of lead borosilicate-typeparticulate material and at least one metal oxide selected from thegroup consisting of magnesium oxide, yttrium oxide, antimony oxide,tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide,neodymium oxide, samarium oxide, europium oxide, gadolinium oxide,terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thuliumoxide, ytterbium oxide and lutetium oxide, said borosilicate-type glassbeing diffused from the surface of the fired varistor element to insidethe fired varistor element.
 62. The process according to claim 61,wherein the glass particulate material and the at least one metal oxideare mixed to form said mixture and, upon forming, said mixture contains5-30% by weight of boron oxide, 5-30% by weight of silicon oxide,40.0-80% by weight of lead oxide, and 0.1-30.0% by weight of said atleast one metal oxide.
 63. A zinc oxide varistor comprising a firedvaristor element having opposite surfaces and at least two electrodesformed on said fired varistor element from an electrode paste, saidfired varistor element comprising a lead borosilicate-type glassdiffused into at least one surface of said fired varistor element dininga heating operation employed to form said electrodes; said leadborosilicate-type glass comprising a mixture of lead borosilicate-typeglass particulate material and at least one metal oxide selected fromthe group consisting of magnesium oxide, yttrium oxide, antimony oxide,tellurium oxide, lanthanum oxide, cerium oxide, praseodium oxide,neodymium oxide, samarium oxide, europium oxide, gadolinium oxide,terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thuliumoxide, ytterbium oxide and lutetium oxide, said lead borosilicate-typeglass being diffused from the surface of the fired varistor element toinside the fired varistor element.
 64. The zinc oxide varistor accordingto claim 63, wherein the glass particulate material and the at least onemetal oxide are mixed to form said mixture and, upon forming, saidmixture contains 5-30% by weight of boron oxide, 5-30% by weight ofsilicon oxide, 40.0-80% by weight of lead oxide, and 0.1-30.0% by weightof said at least one metal oxide.